These icons are a relic of GLabel and were never implemented to
accomodate both image and text. This convenience can always be added
in the future, but no current instance assumes or needs it, so let's
replace them all with ImageWidget to show clearer intent.
Add option to reverse primary and secondary buttons in Mouse Settings.
- WindowServer.ini: add default entry
- switch-mouse-buttons.png: new icon for settings entry
- Mouse.gml/MouseWidget.*: new settings dialog
- ClientConnection/WindowManager/Server: window message for settings
- EventLoop.cpp: swap buttons 1 and 2 if settings are on