osdWindow: scale by scaleFactor

Since we calculate this dimensions from the size of the monitor, the
result was too large. Scale it by scaleFactor instead.

https://bugzilla.gnome.org/show_bug.cgi?id=724607
messageTray: don't force a size on SourceActor's iconBin

Supports hidpi icons this way.

https://bugzilla.gnome.org/show_bug.cgi?id=724607