give user option to click on background to unselect all items

[dave-doty]

I personally would prefer not to use this option, so that I don't accidentally unselect a large group that takes a long time to reselect. But some users may expect this behavior. Currently, pressing Esc unselects everything.

This should be a Boolean option under the View menu. If enabled, then clicking anywhere in the design without the Shift or Ctrl key pressed should dispatch a SelectionsClear action. The Esc button should still also do this.