Add xdg-desktop-portals support

Some GTK-applications, when run in KDE, may use xdg-desktop-portals to display system file open/save dialogs instead of GTK dialogs.
To do this, just set the environment variable GTK_USE_PORTAL=1 before launching such an application.
However, as far as I understand, RDM does not support portals.
Therefore, setting this variable does not change anything - RDM continues to use GTK-dialogs.
Please add support for xdg-desktop-portals.
GTK-dialogs are poor and primitives.