Debugger: Remove direct entering new watchers in the treeview

Instead pop up the "Add new expression dialog". Same amount of activity
needed, but more uniform in UI and code and hopefully less confusion
about the now-gone <Edit> marker.

Change-Id: I228801dc51f6d09ea9991b98399dc9ef04aa96c8
Reviewed-by: Christian Stenger <christian.stenger@digia.com>