Implement Qt's What's This? in Xi-CAM

[ihumphrey]

Qt provides a very useful What's This? tool for allowing users to interactively find short documentation for widgets (see QWhatsThis).

Xi-CAM would benefit from having this tool added to the "Help" menu:

An alternative way to enter "What's This?" mode is to call createAction(), and add the returned QAction to either a menu or a tool bar. By invoking this context help action (in the picture below, the button with the arrow and question mark icon) the user switches into "What's This?" mode. If they now click on a widget the appropriate help text is shown. The mode is left when help is given or when the user presses Esc.