osdWindow: add setMonitor() to allow changing the monitor

This is also exposed in the ShowOSD DBus method, the "monitor"
parameter may contain an integer to indicate the monitor number.
If the value is not provided or <0 is used, the monitor is shown
on the primary monitor as usually.

This way, the OSD can be used to notify upon events that solely
apply to one monitor, like tablet mapping as discussed in
https://bugzilla.gnome.org/show_bug.cgi?id=710373.

https://bugzilla.gnome.org/show_bug.cgi?id=712664