Fix #15162 (GUI: update icons)

Add @2x and @3x icons rendered from the SVG sources of Tango 0.8.90
(the latest Tango release) so the icons are sharp on high-DPI screens.
The 22x22 icons are unchanged. Qt picks the @nx files automatically.

Icons that were renamed copies of Tango icons now use the Tango files:
- showerrors.png -> dialog-error.png
- showwarnings.png -> dialog-warning.png
- showperformance.png -> utilities-system-monitor.png
- openproject.png -> folder.png (22x22 instead of a scaled 32x32)
- showstylewarnings.png was unused and is removed

Hand-made icons are replaced:
- "Reanalyze modified files": Refresh file.svg from the Tango project
  on Wikimedia Commons (public domain)
- "Show Scratchpad": Tango accessories-text-editor

Update the Tango link in AUTHORS, tango.freedesktop.org is gone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: dmcppcheck

AUTHORS

@@ -466,4 +466,5 @@ Zhu Lei
 Дмитрий Старцев
 
 GUI graphics courtesy of Tango Desktop Project:
-http://tango.freedesktop.org
+https://gitlab.freedesktop.org/tango/tango-icon-theme
+https://commons.wikimedia.org/wiki/File:Refresh_file.svg

gui/gui.qrc

@@ -2,31 +2,67 @@
     <qresource prefix="/">
         <file>cppcheck-gui.png</file>
         <file>images/dialog-error.png</file>
+        <file>images/dialog-error@2x.png</file>
+        <file>images/dialog-error@3x.png</file>
         <file>images/dialog-information.png</file>
+        <file>images/dialog-information@2x.png</file>
+        <file>images/dialog-information@3x.png</file>
         <file>images/dialog-warning.png</file>
+        <file>images/dialog-warning@2x.png</file>
+        <file>images/dialog-warning@3x.png</file>
         <file>images/edit-clear.png</file>
+        <file>images/edit-clear@2x.png</file>
+        <file>images/edit-clear@3x.png</file>
         <file>images/go-down.png</file>
+        <file>images/go-down@2x.png</file>
+        <file>images/go-down@3x.png</file>
         <file>images/help-browser.png</file>
+        <file>images/help-browser@2x.png</file>
+        <file>images/help-browser@3x.png</file>
         <file>images/media-floppy.png</file>
+        <file>images/media-floppy@2x.png</file>
+        <file>images/media-floppy@3x.png</file>
         <file>images/preferences-system.png</file>
+        <file>images/preferences-system@2x.png</file>
+        <file>images/preferences-system@3x.png</file>
         <file>images/process-stop.png</file>
+        <file>images/process-stop@2x.png</file>
+        <file>images/process-stop@3x.png</file>
         <file>images/text-x-generic.png</file>
+        <file>images/text-x-generic@2x.png</file>
+        <file>images/text-x-generic@3x.png</file>
         <file>images/view-recheck.png</file>
-        <file>images/view-refresh.png</file>
-        <file>images/showerrors.png</file>
-        <file>images/showstylewarnings.png</file>
-        <file>images/openproject.png</file>
-        <file>images/scratchpad.png</file>
-        <file>images/showwarnings.png</file>
-        <file>images/showperformance.png</file>
+        <file>images/view-recheck@2x.png</file>
+        <file>images/view-recheck@3x.png</file>
+        <file>images/refresh-file.png</file>
+        <file>images/refresh-file@2x.png</file>
+        <file>images/refresh-file@3x.png</file>
+        <file>images/folder.png</file>
+        <file>images/folder@2x.png</file>
+        <file>images/folder@3x.png</file>
+        <file>images/accessories-text-editor.png</file>
+        <file>images/accessories-text-editor@2x.png</file>
+        <file>images/accessories-text-editor@3x.png</file>
         <file>images/utilities-system-monitor.png</file>
+        <file>images/utilities-system-monitor@2x.png</file>
+        <file>images/utilities-system-monitor@3x.png</file>
         <file alias="COPYING">../COPYING</file>
         <file alias="AUTHORS">../AUTHORS</file>
         <file>images/go-home.png</file>
+        <file>images/go-home@2x.png</file>
+        <file>images/go-home@3x.png</file>
         <file>images/go-next.png</file>
+        <file>images/go-next@2x.png</file>
+        <file>images/go-next@3x.png</file>
         <file>images/go-previous.png</file>
+        <file>images/go-previous@2x.png</file>
+        <file>images/go-previous@3x.png</file>
         <file>images/applications-development.png</file>
+        <file>images/applications-development@2x.png</file>
+        <file>images/applications-development@3x.png</file>
         <file>images/applications-system.png</file>
+        <file>images/applications-system@2x.png</file>
+        <file>images/applications-system@3x.png</file>
         <file>images/llvm-dragon.png</file>
         <file>images/verify.svg</file>
     </qresource>