Initial GTK3 support for GP

This adds support for building GP against GTK3 Geany to the Autotools build system.  It adds the various required checks to determine which GTK version to use depending on which the Geany to build against itself uses; as well as adjusting the various check performed by each plugins.

Additionally, it also adds support for GTK3 in some plugins that required adjustments (*WebHelper*, *GeanyGenDoc*).  However, many plugins are left to port to GTK3 though (only *CodeNav*, *Commander*, *GeanyExtraSel*, *GeanyGenDoc*, *GeanySendmail*, *GeanyPG*, *GeniusPaste*, *PoHelper*, *ShiftColumn*, *Tableconvert*, *Updatechecker*, *WebHelper* and *XMLSnippets* are currently building against GTK3 -- and few of them were really tested with it).

After this gets merged, plugin authors will be encouraged to add GTK3 support for their plugin.