VisuStruct 1.2.3

Release after 1.2.2.

Kotlin migration

Production sources are now Kotlin under src/main/kotlin/ (UI, control, structure elements, code generation, simulation XML).
Former Java sources are kept for reference only in src/main/java-reference/ (not compiled).
Tests remain Java under src/test/java/ and run against the Kotlin main code.
Behaviour and file format unchanged: .visustruct / XML projects remain compatible.

Element editor and common dialogs (zoom, colour, font, element edit) use bundle keys (editor.*, dialog.*) with EN + DE and EN fallback in all 21 UI locales.
tools/resync_message_bundles.py rebuilds locale files from Messages_en.properties when VisuStruct-swift is not available locally.
tools/translation_aliases.json maps new keys from existing translated Java keys where possible.
Full Swift parity still requires a sibling VisuStruct-swift clone and python3 tools/sync_java_messages_from_swift.py.

Element editor split pane: one-touch expandable divider (editorSplit).