Adding alternate mode (alt + left click) for panning

The rubber band 2D interaction mode, used by many infovis views,
did not have a mechanism for panning with a 2-button mouse or
trackpad. The alt + left click combination makes this possible again.

Change-Id: Ice82b01e98795230dea02a945a7d3659bb111114
Merge topic '2d-alternate-pan' into master

d7c0f611 Adding alternate mode (alt + left click) for panning