Some GUI stuff

- Settings now a drop-down menu, new option 'CKAN settings' replaces the
old one
- KSP command-line options now under 'Settings'
- Fixed a bug where the mod list columns wouldn't get resized properly
on repo update
- KSP executable path now configurable from 'KSP command-line'
- User now gets prompts for 'suggested' mods
- Other minor fixes

Closes #444
Closes #326 
Closes #284
As a note, it's probably better if we chdir to the gamedir first, see #451 .