Fullscreen windows with WS_EX_NOACTIVATE | WS_EX_TRANSPARENT interfere with UI events

This is the GTK+3 port of a patch we have been applying on GTK+2 built for GIMP's Windows installer for the last 3 years (cf. gimp#1082 (closed)). Basically many applications apply an invisible window over the whole desktop (screencast software, video conference, gesture-triggered actions, etc.) which interfere with mouse events and break various GUI interactions of GTK applications on Windows. This issue has been a huge problem (we had many reports about this) though we have had very few (apparently there may exist some other similar issues though) ever since this patch got applied. Since Inkscape had the same issues, even on GTK+3, they contributed it on the MSYS2 package of GTK+3 (which is the main package used by projects building GTK software for Windows) as it was confirmed to fix bugs reported to them as well.

I think it makes sense that this patch which has been now used for years on built packages and proven to work be committed upstream, no? 🙂

This should fix #370 (closed) or at least some cases of a more generic issue.