Refactor config.h handling.

- Use separate config.h files for kate and kwrite.
- Add `KWRITE_VERSION` for use in kwrite source code.
- Use the more widely used .in suffix for config.h templates instead of
the .cmake suffix which is primarily used by CMake script files.
- Rename `KActivities_FOUND` define to `KF5Activities_FOUND` so that
cmakedefine in config.h.in automatically picks it up when
`KF5Activities_FOUND` is set during CMake's configure stage.
- Remove config.h include from source files that don't need it.