Tidy up: Remove pylint references.
Weather: Create signal before running threads.

Create the signal before running functions in another thread that
attempt to emit the signal.
Monitor Control: Add an initial class.

At the moment, it controls the DPMS functions of the monitor.
Menu: Add button to turn monitor off.
Thread: Create threaded base class for single thrd

The new base class ensures that all objects of a class derived from
ThreadedClass share a single thread.
This is to prevent the usual problems associated with threaded apps accessing
a single resource, and to prevent a build up of dead threads.
PhotoManager: Write the rating to image's exif.

The exif writing is done with either pyexiv2 or gexiv2, depending on
which library is present (since pyexiv2 is deprecated, gexiv2 is
preferred). All I/O operations are run in a seperate thread.