GUI

Transform the CLI into a GUI:
- [x] Main window
  - [x] Currently installed mods
  - [x] Mods to be installed
  - [x] Sync button
  - [x] "Uninstall all" button
- [x] Sync window
  - [x] Button to abort installation
  - [x] Log panel
- [x] Error Handling

For later:
-  Progress bar (with weight based on mod size?)
- Pretty pictures
- #8