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.