Switch chart selection modifiers

This switches the chart selection modifiers. Now ctrl maps
to plus and shift maps to minus. This more closely matches
the controls in ParaView's 3D render view.

Change-Id: Id42ba06404ac795ad4a4b85cd5896cb0d890b165
Merge topic 'switch-chart-selection-modifier-keys' into master

9ec9e70c Switch chart selection modifiers