Implement a tree diff, that allows to compare directory trees

[matthiasblaesing]

Usage: Select exactly two folder entries in the project/files/favorites, open the context menu and select "Tools" -> "Tree diff"

This results in a flattened view of the difference between the two trees:

• Entries with red background are present in the source, but not in the target
• Entries with blue background are present in both locations but differ
• Entries with green background are present in the target, but not in the source

The list of modified files/folder can be display as hierarchy:

When a modified file is selected the diff view shows both versions and allows to update the target:

Additionally actions can be invoked directly on the difference entries:

Closes: #8349

[matthiasblaesing]

As this has little side effects and would be trivial to remove, I intent to merge at the end of week, unless someone objects.