Add: Create AreaPickingEvent to handle area picking
Update: add ability to handle area picking events

Make PCLVisualizerInteractorStyle inherit from vtkInteractorStyleRubberBandPick
Add area picking callback and signal
Add switch to enable area selection (r or R)
Add switch to enable area picking (x or X)
Pass the super class selection mode to the mouse event
Remove trailing white spaces
Add: a flag to see whether we are in select mode or orient mode
Add: vtkPointSource.h header
Add: performAreaPick method to get point indices in the selected area

Update: take into account the mouse event switch which indicates the current mode
Update: add area picking feature to PCLVisualizer

Add ability to register an area picking callback
Use a vtkAreaPicker instead of vtkPointPicker
Add: include area_picking_event.h header
Clean: remove extra headers
Merge remote-tracking branch 'upstream/master' into area_picking