Python GUI code improvements

[Lgt2x]

The Python files in PM-GUI need to be re-worked to be understood and edited more easily.

The current "module" (which is not one yet) is split in 3 parts (per @kloptops ):

• pugwash - most of the glue that binds the gui to the package manager
• pylibs/pugthemes.py - the craziest theme engine
• pylibs/pugscenes.py - the actual different scenes that are presented

Various improvements that I have from the top of my head:

• Make PM-GUI a real Python module, not using crazy relative imports
• Remove all usage of __builtins__ to set global variables. Prefer dependency injection and/or a shared settings object to global variables
• Use parseargs instead of manually parsing string arguments
• Standardize Python version and module requirements
• Setup minimal automated tests, at least testing that the UI launches
• make pretend_device a CLI argument
• Factor code between pugwash and harbourmaster

[kloptops]

Yeah these are good suggestions.

I have been meaning to have the modules in exlibs sourced, this would also be appropriate to be a point in the build/release process restructure.

Python version is dependent on the CFW we are running on, thus all the modules i have used are very flexible with the version of python they will run on, which at the time of writing is Python 3.7.5.