disallow group at root of tree

[westram]

Currently it is possible that a has group-information at the root node:

• example from userland: ~/data/test_tree_with_rootGroup.arb
• minimal example DB​

Reproduce creation:

• switch to GROUP MODE
• right-click outside the
  • enter the name of the new group (e.g. 'top')
• the will display as single collapsed group.
  The name of the group ('top') is not displayed, instead the treename is shown.

Problems:

• the group at -root behaves weird:
  • it will always stay at root (even if root is moved around)
  • it cannot be moved (in MOVE MODE)
    • but it can be swapped with any other group (in MOVE MODE). In that case the given groupname ('top') shows up again.
• general problems
  • a group at root has no father-node (any other group has one)
  • such a group is semantically "outside the "

Wanted fixes:

• eliminate such groups when gets loaded ("into" display) by [15874]
• avoid such a group will be generated:
  • directly as described in 'Reproduce' above
  • indirectly during load (root-group persists save+load)

---

Related: #451, #735

[westram]

example

[westram]

• implements workaround (=removed group at load)