Add support for importing arbitrary filters

[Apollys]

There are various approaches and levels of complexity this could take.

Easiest approach: add-only

• Add all DLF functionality as custom rules to the beginning of the filter
• Downside: may override original filter functionality in undesirable ways

More complex approach: search and add if missing

• Search for specific rules on import, if not found, add rule to beginning of filter
• Save all tags for reference later

Universal tag approach:

• Collaborate with filter designers to create a universal tag standard
• Can then parse filters based on these universal tags
• Downside: requires extra work from filter designers at little benefit to them