plasma volume: global shortcuts and osd

[Harald Sitter]

random update before akademy.

d_ed asked me to perhaps land the volume thingy for 5.4 so to make
that happen the applet UI is going to stay minimal and the KCM UI is
going to stay a clone of the pavucontrol UI (which isn't the greatest
of things but functionally pretty complete).
both UIs should be more or less complete in master of
scratch/sitter/plasma-volume-control. I encourage testing.

also I added global shortcuts and OSD support which should be the two
major features that were missing. also kindly note that no measures
are being taken to replace kmix right now so with kmix running you'll
get two OSDs, other than that everything should be fine.

HS