rqt_reconfigure: Add context menus for common operations

I added some quick 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.

For the most part, this addresses (https://github.com/ros-visualization/rqt_common_plugins/issues/211)

Feedback is appreciated!

--scott
For consistency with the rest of the file I would suggest using single quotes (`'`). Just a comment - doesn't have to be changed...