Fixes #11836.  Patch from avoidscorn (CLA).  _FormWidget onMouseDown now only consumes a left mousedown.  Added automated contextmenu test.
Fixes #11836.  Patch from avoidscorn (CLA).  _FormWidget onMouseDown now only consumes a left mousedown.  Added automated contextmenu test.

git-svn-id: 674a2d6b1e32e53f607eb824547723f32280967e@23303 560b804f-0ae3-0310-86f3-f6aa0a117693

Sam-Cummins/delite,kitsonk/delite,Sam-Cummins/delite,kitsonk/delite,harbalk/delite,harbalk/delite