Closes GH-301 Remove core from apps that don't need it

Core.py is no longer required. Tools should import core modules that they
need:

- config
- database
- logger
- multimedia
- plugin

Renamed some modules:

- QtDBConnector -> database
- gstreamer -> multimedia

Moved Core.DB functions to the database module
Moved Core.Backend functions to the multimedia module
Added settings module which stores useful global variables
CLI now only displays OS specific plugins

Closes GH-271 CLI should only show os-specifc plugins