Prompt to delete dirs containing empty registered dirs

[HebaruSan]

Problem

1. Install TweakScale with the current dev build (problem reported not to happen with the released version)
2. Launch the game
3. Start or load a save
4. Launch a rocket
   This (or the previous step, not clear) should create a GameData/TweakScale/Plugins/PluginData/Scale/config.xml file
   
   (Don't ask me why this isn't just GameData/TweakScale/PluginData/config.xml instead.)
5. Close the game
6. Uninstall TweakScale
7. You're prompted to delete the PluginData folder, but not the Plugins or Tweakscale parent folders
   
8. As you'd expect, clicking Delete Checked leaves the TweakScale folder there

Cause

I think #3890's changes to the the deletion logic must have affected how subdirectories are treated, but I was more focused on understanding and fixing the current logic than on the details of the history.

Changes

Now when we check whether to ask the user about deleting a directory, we ignore any subdirectories that are already in that list. This will add the GameData/TweakScale and GameData/TweakScale/Plugins folders to the list, and clicking Delete Checked will result in a clean GameData:

• I'll adjust the auto sizing of the help label before merging, just forgot to take care of that before pushing

Fixes #3936.

[techman83]

Clean and concise, awesome @HebaruSan !