Add compatibility wrappers to geany.h for newer GTK+ functions

Does this look like a reasonable way to support building with `-DGSEAL_ENABLE` while still working with older versions of GTK?

I removed `interface.c` from the build system while checking the build errors for `-DGSEAL_ENABLE`, and wherever there was an error I either used the equivalent GTK+ 2.16+ function(s) or added a wrapper in `geany.h` and used that.

There at least one `-DGSEAL_ENABLE` failing usage in `notebook.c:267` where I added a `FIXME` comment because I'm unsure how to fix it.