Note to other developers: you might have to do `make clean;./autogen.sh`
after this commit.

* Use a GtkCheese namespace to be constistent with other vapi's (such as
GtkClutter).

* Put each library in a separate vapi-file (this will come in handy when
porting to Meson).

* Clean up config.vapi:
  * get rid of unused CCode prefix-attribute (removes warning)
  * be consistent with braces