Bulk renamer: Improve UI/UX on rename error

Currently it can happen that the apply button is grayed / cannot be pressed, but the user has no idea why. (E.g. see #202 (closed))

The idea is to improve the UI of the bulk renamer with some of the suggestions below:

1. Scroll to the first occurrence of conflict
2. A tooltip on the 'apply' button could tell that it cannot be pressed because of conflicts.
3. It would be nice to have a tooltip on each conflicting new name, which tells what exactly is wrong for that new name.
4. Add GtkIconBar with some error message and show it in case of conflict. Possibly the concrete error of the first conflict and the number of conflicts could be shown.