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
Hmm, it looks like on Linux this is calling `xdg-open`, which won't open a binary file; although I haven't looked into this deeply. (In my case, it pops up the file browser for the KSP directory). Also, are these default config options that users can modify later? On my system I need to have an argument *before* KSP to run it on my discrete graphics card (`optirun ./KSP.x86_64`). Likewise, I'm sure some users may not wish to run with `-force-opengl`.