file chooser: Move blocking I/O to a thread
file chooser: Move blocking I/O to a thread

When our call to the backend returns, we do some sync calls
to the document portal and the permission store to register
the returned files. We should not do that on the main thread;
move it all to a task.

matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal