Remove unused GTypes for enums, and declarations from the thumbnailer

... by splitting the generated type definitions into separate files.

Currently all the generated GTypes for enumerated types were being added to the thumbnailer, even though the thumbnailer didn't need them.

This was made worse by the fact that the enumerated types were defined in header files that declared new GObject classes or functions. So, when these header files were included to register the GTypes, it also pulled in those other declarations in them.