gui_utils: unpack assets on demand and better starbound folder selection

This PR changes to behaviors of the initial setup:
* starbound folder selection
  * catch incorrect folder selection earlier
  * loop folder selection until correct folder is selected
  * look for starbound.config as correct folder check
* unpack assets on demand
  * check if assests are unpacked
  * if not: show an error promt and offer the user an option to do it within starcheat