Editors can mess up the slug accidentally

[franzkugelmann]

We experienced issues when editors did their work (TYPO3 10 - sluggi 2.2.0).
Here is a series of screenshot for the defect editing process:

1. The admin sees the slug correctly. The slug is NOT representing the pagetree, but shortened for a specific part of the page:
   
2. The editor does not see the slug correctly (the tree path instead of the real slug is shown):
   
3. When starting to edit, the slug looks fine, but after saving the prepending slash gets lost.
   (EDIT: this seems to be fixed already but was mentioned yesterday an T3UD as issue, if that still occures somewhere)
   However, when recalculating, the first part of the page title gets lost and more severely the current path that the editor should NOT be able to change gets lost, creating a new root-slug.

#73 fixes those issues, slugs are correctly shown to the editor and also saved correctly.