First refactoring of the pivoting
Heights / depths calculated for rearranging the pivot thread in view

- The idea is to draw a horizontal path instead of a vertical one
Further work on the pivoting

- still has some issues with arranging the height for some branching
Finished the first version of the new pivoting

- Users can go back to a previous event and branch the pivoting by choosing a new relation

- users can remove individual pivoted branches
Fix to removing the root element causing issues with pivoting
Height adjustment was not cummulative

- inserting a branch to a previous sibling only pushed the next sibling down a line, not the following one. Fixed.
Delete button gone from pivot elements that should not be deleted

- When looking at an event, a user should not be able to delete the pivot path that he/she took to get to that particular event.

- Deleting the root pivot item is an exception, this will simply reset the pivoting.
Merge branch 'hotfix-2.1.19'