rqt_reconfigure: Add context menus for common operations

All widgets:
- Set to default
Integer and Double:
- Set to Minimum
- Set to Maximum
Double:
- Set to NaN
String:
- Set to Empty String

Slider bar is reset to default when widget is set to NaN instead
instead of crashing
Merge pull request #251 from cottsay/context_menus

rqt_reconfigure: Add context menus for common operations