ENH: Add a control-select modifier for selection toggling.
ENH: Add a control-select modifier for selection toggling.

This allows the user to hold down the control key while performing a drag
selection. The points selected will be toggled (selected points deselected
and unselected points selected).

Change-Id: I6d2ffdde05101e846dd91f90adf015079bb4a3b8

johnkit/vtk-dev,ashray/VTK-EVM,mspark93/VTK,gram526/VTK,biddisco/VTK,gram526/VTK,sankhesh/VTK,ashray/VTK-EVM,jmerkow/VTK,ashray/VTK-EVM,msmolens/VTK,biddisco/VTK,berendkleinhaneveld/VTK,berendkleinhaneveld/VTK,sumedhasingla/VTK,gram526/VTK,demarle/VTK,SimVascular/VTK,candy7393/VTK,gram526/VTK,collects/VTK,sumedhasingla/VTK,demarle/VTK,msmolens/VTK,aashish24/VTK-old,sankhesh/VTK,berendkleinhaneveld/VTK,biddisco/VTK,msmolens/VTK,demarle/VTK,gram526/VTK,ashray/VTK-EVM,gram526/VTK,cjh1/VTK,biddisco/VTK,aashish24/VTK-old,spthaolt/VTK,msmolens/VTK,spthaolt/VTK,sumedhasingla/VTK,berendkleinhaneveld/VTK,sankhesh/VTK,collects/VTK,candy7393/VTK,jmerkow/VTK,jmerkow/VTK,sumedhasingla/VTK,gram526/VTK,candy7393/VTK,aashish24/VTK-old,msmolens/VTK,candy7393/VTK,biddisco/VTK,johnkit/vtk-dev,sankhesh/VTK,mspark93/VTK,spthaolt/VTK,demarle/VTK,hendradarwin/VTK,keithroe/vtkoptix,berendkleinhaneveld/VTK,ashray/VTK-EVM,hendradarwin/VTK,SimVascular/VTK,keithroe/vtkoptix,sumedhasingla/VTK,cjh1/VTK,sumedhasingla/VTK,sumedhasingla/VTK,cjh1/VTK,spthaolt/VTK,msmolens/VTK,demarle/VTK,gram526/VTK,SimVascular/VTK,SimVascular/VTK,candy7393/VTK,biddisco/VTK,hendradarwin/VTK,keithroe/vtkoptix,candy7393/VTK,candy7393/VTK,collects/VTK,mspark93/VTK,sankhesh/VTK,jmerkow/VTK,johnkit/vtk-dev,cjh1/VTK,msmolens/VTK,SimVascular/VTK,sankhesh/VTK,cjh1/VTK,ashray/VTK-EVM,demarle/VTK,hendradarwin/VTK,jmerkow/VTK,demarle/VTK,hendradarwin/VTK,ashray/VTK-EVM,johnkit/vtk-dev,keithroe/vtkoptix,collects/VTK,hendradarwin/VTK,collects/VTK,keithroe/vtkoptix,aashish24/VTK-old,jmerkow/VTK,johnkit/vtk-dev,mspark93/VTK,ashray/VTK-EVM,spthaolt/VTK,keithroe/vtkoptix,mspark93/VTK,spthaolt/VTK,candy7393/VTK,aashish24/VTK-old,sankhesh/VTK,jmerkow/VTK,keithroe/vtkoptix,berendkleinhaneveld/VTK,collects/VTK,demarle/VTK,mspark93/VTK,msmolens/VTK,mspark93/VTK,sumedhasingla/VTK,hendradarwin/VTK,johnkit/vtk-dev,spthaolt/VTK,aashish24/VTK-old,keithroe/vtkoptix,biddisco/VTK,SimVascular/VTK,mspark93/VTK,johnkit/vtk-dev,jmerkow/VTK,sankhesh/VTK,SimVascular/VTK,berendkleinhaneveld/VTK,SimVascular/VTK,cjh1/VTK