Open and close repos via Main UI

- Add slots for opening and closing a local git repository
  - Connect action signal for opening a repo to it's corresponding slot
  - Connect signal for closing a canvas tab
- Switch from loading .ui file for main window dynamically to using a
  python module generated with the pyuic utility (to access ui elements
  from main.py that were laid out in Qt Designer)
- Add a makefile for generating Python modules from .ui files
- Refactor ui folder structure (still needs some more love)
- Remove test data from main.py