Add hook to override QPlatformTheme::SystemIconThemeName for app

I would love to see an option making Qt able to change

QPlatformTheme::SystemIconThemeName

on its own, without having to build Qt with Glib/GTK support.

Such an implementation could be done for QtQuickControls directly inside the qtquickcontrols2.conf file, with something like

[Controls]
Style=Material
 
[Material]
Theme=Light
Accent=Teal
Palette\Window=#000
Icon\ThemeName=Breeze

Then the application would look inside $XDG_DATA_DIRS/icons or $QT_QUICK_CONTROLS_ICON_PATH to find the right icon theme.