Provide option to overwrite the dragged files

My preferred workflow is to drag the icons I want to use from the
gallery into Xcode, then change colours in place.

This option allows me to drag files to the colour changer, and not have
to select the destination folder.
save colour in a preference

The colour is automatically saved via binding.
The preference define in GCCIMportView.h is just in case someone wants
to access this programatically in future
Adjust min/max size

ensures that the overwrite preference is visible after pressing
maximise.
Removed ability to resize window and changed overwrite text
Changed version
Fixed some personally preferred coding conventions
Merge pull request #6 from glyphish/pr/5

Closes #4 and #5.  Merges #5 with additional commits.