Add explicit 'close' button to region selector

[factormystic]

The UI is too minimalist, and the feature needs love:

• A close button/link
• Resize targets are too small/fiddly
• Esc/Print Screen should both have context relevance for this
  • Mouse needs to not be required to use this
  • Should there be a new section of shortcut keys for the region selector?
• Interior dimensions display
• Average color information display
  • Histogram?
  • This may be more appropriate for the editor