Change mpris play/pause button color instead of removing it

Summary:
This prevents issues with buttons jumping around when canPlay/canPause is false, as some mpris players do not report consistent values.

Additionally, gives the play/pause button a bit more attention when enabled. What do you think?

When playing/pausing is enabled (i.e. nearly always):

When disabled:

Reviewers: KDE Connect, VDG, albertvaka

Reviewed By: KDE Connect, albertvaka

Subscribers: albertvaka, nicolasfella

Differential Revision: https://phabricator.kde.org/D10796