add platform-specific default.qss

[ronso0]

default-mac.qss
default-linux.qss
default-windows.qss

Right now there's a lot of redundancy across skins in style sheets for most Mixxx menus like WTrackMenu and #MainMenu menubar; and for complete theming styles for the Track Poperties window, tag fetcher and Preferences will be added, too.

To ease maintenance of all that let's move the common proven styles to a central place.

[ronso0 (ronso0)]

that was easy, implemented here
https://github.com/ronso0/mixxx/tree/platform-default-styles

put on hold for now (2.3) because
* for custom skins make the main menubar hard to use because _any_ property set for #MainMenu would remove the OS palette, thus there would be no selection highlight and no checkmark icons
* Mixxx skin's menu styles make use of '#LibraryContainer QMenu...' (shortcut to select QMenus in WLibrarySidebar , WLibrary, WSearchlineEdit, ...) and that doesn't pass the qsscheck pre-commit hook (yet) because #LibraryContainer is not set anywhere in c++ and is probably not used in custom skins, thus custom skins would have inconsistent, broken menus

[Daniel Schürmann (daschuer)]

Can this one be closed?

[ronso0 (ronso0)]

not really.
recently I introduced default-menu-styles-[windows, linux].qss, though those are not parsed by the skinloader like default.qss but just set in the Style node of skin.xml via src-win and src-linux.
for mac we still need custom stylesheets to work around some strange click/release behaviour (skin setting buttons in Deere and Tango).
That said, it would still be convenient to have the requested platform styles parsed without having to specify them in each skin.xml

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/10393