Implemented a first version of an interactive
screenshot dialog that supports annotations, text excerpts
for these and allows to create a screenshot file

How to use:
* enable experimental features in Tools->Options
* open any random dialog
* open context menu - there's now a 'Screenshot' entry
* click that, screenshot dialog pops open, mouse over the
  grayed-out screenshot to highlight controls
* click once to highlight, click a 2nd time to unselect

Change-Id: I3bcd76b96ad023e11421e4fcfac866ebf4f5ff78