In Windows builds of GTK+ 3.x, the dll filename for GDK is
libgdk-3-0.dll.
We use this opportunity to use a common const in the DllImport for all
custom code.
Also:
* improving naming consistency
* obsolete old/broken stuff
* clean up tabs/space mix
NOTE: this removes the wrapper caches for Pattern and Surface as
there was no reliable way to clear them.
This is a merge from changes in Mono.Cairo in mono 3.2.