New Application Icon for vis (& Other Things)

- New application icons in 32, 64, 128, 256, and 512 approximate-square
  sizes. All PNG, in /views/ui/icons/vis-1-size.png
- Streamlining of main_window
- Removed two debugging 'print' calls
- Recompiled icons resource file
- Revised controllers/importer.py

Closes #99