Blender Git Loki

Fix T39234: popup menus behave poorly when they have not enough width for all their columns.

Issue fixed by:
* Not having constant width for all columns, but adapt each to its content's width;
* Adapting undo's menu height to undo list length (so that we never have more than three columns).

It is still possible to get issues in extreme cases (small screen, high DPI size,
long op names everywhere...), but this should now be rare corner cases.

Also fixes a minor glitch with undo menu (first column had one item less than the others...).