Add macros to check for the GTK version to build against
Check for the GTK version to build against
commander: Add support for GTK3
GeanyGenDoc: Add support for GTK3
WebHelper: Add support for GTK3
Disable plugins that check for a dependency on an incompatible GTK
Mark plugins that don't build on GTK3 as requiring GTK2

Plugin authors should remove or adapt this check when they port their
plugin to work with GTK3.
Show in the status the GTK version the Geany we build against uses
Merge pull request #91 from b4n/gtk3-support

Initial GTK3 support for GP