Cleanup: BLF FontFlags enum type safety, move enums to separate header

- BLF font flags were an untyped enum; change to actual enum and
  update usages from int to that.
- Move all BLF API enums to their own header, so that if something
  needs just the enums they don't have to include whole BLF_api.hh

Pull Request: https://projects.blender.org/blender/blender/pulls/143692

source/blender/python/generic/blf_py_api.cc

@@ -324,7 +324,7 @@ static PyObject *py_blf_disable(PyObject * /*self*/, PyObject *args)
     return nullptr;
   }
 
-  BLF_disable(fontid, option);
+  BLF_disable(fontid, FontFlags(option));
 
   Py_RETURN_NONE;
 }
@@ -349,7 +349,7 @@ static PyObject *py_blf_enable(PyObject * /*self*/, PyObject *args)
     return nullptr;
   }
 
-  BLF_enable(fontid, option);
+  BLF_enable(fontid, FontFlags(option));
 
   Py_RETURN_NONE;
 }