Adds a blender-style bit mask selector in the editor for 'Mask' attributes.
- Changed the method to determine whether an attribute should be made into a bit selector editor.
- Bit masks outside of the selection range cause bit selectors to gray out to avoid confusion.
Merge remote-tracking branch 'hdunderscore/Editor_BitMaskSelector'
For Travis CI - switch CI build to use Raspberry-Pi build environment.