#1920 GTK: incorrect popup placement on secondary monitors

[Colomban Wendling]

Fix calltip and other popups placement when the monitor they are on isn't located at 0,0 screen coordnates. See also https://github.com/geany/geany/issues/1422

In [4fcd47d43161cfe072cdb355a19b4deef7206322] there is some screen vs. monitor confusion, leading to incorrect placement when the monitor has an offset, which typically happens on the second monitor of a two-monitor setup.

Attached patch fixes it properly taking monitor origin into account.

It could be even improved further to get the monitor at the exact expected popup location instead of just where the widget's at, which would improve the situaion where the widget overlaps 2 monitors. But that's outside the scope of this fix.

[Neil Hodgson]

Committed as [8e0cb3].