Fixes issue with selections in charts.

This addresses several bugs with chart interactions and selections including
BUG #14223, BUG #14224, BUG #14225. We now have two buttons for selection types:
rectangular and polygonal. One presses those to enter selection mode (or use the
corresponding key-board shortcuts). The modifier buttons are don't start/stop
selection, but just affect how the selection is updated on the subsequent
selection action. Selection modifier is persistent and hence persists till user
manually changes it. We switch in and out of selection mode similar to what we
do for 3D view i.e. on mouse-up, we come out selection mode to regular
interaction mode.

Change-Id: I0e1c683921f2a7c777ab28d62def4157233b180f
Merge topic 'fix_chart_interactions' into next

d09ef62 Fixes issue with selections in charts.