MultiTerm: Add support for GTK3

This PR adds support for GTK3 by adjusting the build system and updating the GTK3(-stack) API used.

It includes an ugly hack for the `geany.vapi` and `geany.deps` files by simply copying them to new names, replacing and/or commenting-out some incompatible stuff and using the appropriate one decided at build time. I think it's OK for now since we'll be improving/sharing Vala bindings support for plugins soon enough and this part won't be needed once the GTK3 support is done correctly in the future shared bindings.