window: read icon from mount point for AppImage

For AppImage, the icon is usually in the same folder with its desktop file, but the folder is a temporary folder and cannot be found by QStandardPaths::locate(GenericDataDir), so try to manually find it.

See https://docs.appimage.org/reference/appdir.html#general-description

AppImage issue: https://github.com/AppImage/AppImageKit/issues/1231

Before (See the icon in the title bar of osu!)	After

See also: plasma-workspace!2558 (closed)