Join skeletons: fix bug caused by wrong order of variables
Join skeleton: remove IST group handling

Because annotations will replace also groups, no special handling for special
groups is needed anymore.
Join skeletons: write combined annotations to 'from' neuron

The joined skeleton models the neuron modeled by the 'from' skeleton. Therefore
should the combined annotation set by linked to this 'from' neuron. Before, the
annotations were linked to the 'to' neuron. This commit fixes this.
Join skeletons: add better one-node skeleton handling

If the to-skeleton has only one node, it is checked whether is has any
annotations. If not, the node is joined right away. Otherwise, the dialog is
shown. If the to-skeleton has more than one node, the dialog is shown in all
cases.