Make a group when selection contains a group, everything disappears

[lastobelus]

The case where the selection consists of only one element that is a group is handled, but this bug manifets if the selection contains two or more objects one or more of which is already a group.

This may or may not be fixable; not sure if recursive groups are doable in fabric. If not fixable, we should either:

1. do nothing and report a warning if the user tries to make a group with a selection
2. ungroup all the subgroups and make one group with all the objects