Use xdg-desktop-portal to get color scheme used by GNOME. Recent GNOME
versions use this as primary way to switch between light and dark theme.
Make this a preferred way to get color scheme and fallback to previous
methods in case xdg-desktop-portal cannot be used. Also update app theme
on runtime when color scheme changes, not only when theme is changed.
[ChangeLog][Platform Specific Changes][Linux] Add support for
xdg-desktop-portal to get color scheme in QGtk3Theme.
Fixes: QTBUG-116197
Change-Id: Ib3ffad405bc795ed6f4de4af411efc45721662b9
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
Reviewed-by: Santhosh Kumar <santhosh.kumar.selvaraj@qt.io>