Always show resident notifications

Previously, resident notifications were shown only once to make them
appear in messaging area. However, they weren't updated anymore if the
main window is active.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
GNOME resident notification with playback controls

To achieve persistent notification with playback controls in both GNOME
Shell notification area and GNOME lock screen, it is necessary to
add resident flag, set category "x-gnome.music" and keep updating
the notification continuously.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>