[LDPatternCreator] Marquee selection behaviour

[Philippe Hurbain]

There are two things I don't like that much with LDPC marquee selection:

• If I try to extend a selection using ctrl + marquee drag, already selected elements covered by the new marquee get deselected. It would be much more useful if ctrl + marquee only add to selection
  
• In triangle creation mode, if you make a marquee to select a vertex and miss this one, a new vertex is created. It would be better that nothing happened if there is no vertex in marquee area.
  

Otherwise, still longing for an area auto-filler triangulation