[media controller applet] Support MPRIS services where CanPause==false

Summary:
Currently the media controller only works with MPRIS services which support
the PlayPause command. By the MPRIS spec this is an optional feature though.
While most players out there support this, the MPRIS support in Gwenview
currently worked on does not (would need some more rewrite of internals).
Which makes the Media controller applet and/or Gwenview look broken :)

To keep the current design with a central Play/Not Play button, this patch
changes its behaviour depending on the CanPause flag:
CanPause=true -> show either Play or Pause
CanPause=false -> show Play enabled/disabled

Reviewers: broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D10973