Scan rework - preparatory changes

[zoltanvb]

Description

A batch of changes without changing the current behavior. Actual business logic changes will come in another PR soon(ish). The reason to send these earlier is to minimize the main PR content, in case it needs to be reverted.

Even though there are changes in several files, it is not visible - new menu items are not yet being created. But, to give a preview (and a bit of help for translators), it is planned to look like this:

• Scan Method: Automatic (same as previous autoscan) or Custom. Changing to Custom will display the extra options of the old manual scan + a few new ones:
• Database Check: strict (only content that matches DB will be added), loose (all files will be added, with DB reference if possible), custom DAT strict or loose (e.g. "filter DAT contents"), none (no DB scan, same as old manual scan)
• Database To Match: automatic (same as previous autoscan), automatic / first match only, or selected DB
• Target Playlist: old "System Name", it is much clearer to call it that
• Skip DB References: a fallback option if someone wants to match against DB but use own thumbnails directory
• Scan Single File: an option to cover previous "Scan File" functionality

Related Pull Requests

#18458
#18565