Improvements to CDI element support

[bobjacobsen]

• Implements the <action> element to (optionally) provide a button on the CD/CDI panel.
• Implements display hints to recommend that a integer variable be displayed as a slider
• Improve how out-of-range variable are displayed for integer maps
• Fixes a bug in how short <string> variables containing 0x0A newline characters are handled

I'm also working on a <blob> element in overlapping code, so it would be good to get this merged when possible.