plugins: Let plugins fill GeanyPlugin::callbacks instead of passing their own pointer

plugins: Let plugins fill GeanyPlugin::callbacks instead of passing their own pointer

This is easier to handle if we decide to add callbacks. Since we can
zero-initialize callbacks before passing it to the plugin we can be certain as
to which callbacks the plugin knew about when it was compiled. This is exactly
the same method used for GeanyPlugin::info already and easier than inspecting
the API version.