Amend Mechanic to revert or improve imports

[pSpitzner]

First step is the 'undo import' terminal action.

For now I like the current form where we give the user some interaction via the terminal before deleting things from disk and library.

Improvements:

• GUI version of undo import. This would also update the tag-status correctly, which now becomes invalid.
• requires a popup / alert "this will delete folder xyz from your library"
• ideally, notify downstream libraries like plex that files were removed. (i think just triggering a plex scan is not enough)
• add gui_import_id when using terminal imports. this requires adding more tag data in the context menu action and siblings. will only work when terminal-importing a single tag.