sound applet: added qmmp + needed changes for it

Hi,
I've added the support for Qmmp. Most of it was working out-of-the-box, except for a few exceptions, which are probably errors in the MPRIS2 implementation on the plugin author's side; anyway I thought it would be easier for me to fix it here than there.

What I did:
Two conditional changes looking up the player's name mean I'm just extra cautious not to break anything, but the condition itself can probably be omitted altogether (like lowercase value["metadata"]) or changed to support more cases (like the missing "file://" for the artUrl).

Also there was a problem with the timer resetting to 59:59, because qmmp sends "-1000" when it is in a stopped state.
The other slight change NOT related to qmmp was uncommenting two lines in _stopTimer which would reset the timer - I'm sorry if I shouldn't be mixing fix with something that could be considered a subjective change, but while working on the timer it occurred to me, that every other field is set to the default state except for that one case - which was not really consistent.

I'm new to github, so excuse me if the explaining was unnecessary or I've made some other silly mistakes.