Notify user when installing a style whose elements already exist

Related to PHPBB3-9675.

When a style is deleted, its elements may be left in the database. It appears that if a style with the same name is subsequently reinstalled the old elements continue to be used, even if the new style is located in a different directory. This may lead to confusion and broken styles.

Suggestion is to detect the situation when some of the style elements are carried over from the database instead of imported from the style files when a style is being installed, and notify the user about it.