Allow hiding the toolbar for kiosk displays

[jbeers]

Adds a display.toolbar_visible setting for kiosk/thin-client use cases. When disabled, the web viewport uses the full display while controller shortcuts and Settings remain available.

Includes a config parsing test and documentation.

[mxmgorin]

Thanks for the PR, and sorry to turn it down, I don't want a third setting on the toolbar. toolbar_position and toolbar_autohide both go meaningless once the bar is off, and the configurable toolbar gives "no toolbar" for free as "every element at order 0", so this would be a second mechanism for the same thing.

And there is problem on touch: with the bar hidden on a touch-only device there's no way back out - no menu, no settings, recovery only by editing config.toml in app-private storage.

For fullscreen, I'm leaning toward making it a toggleable mode rather than a settings flag.