Rewrite of IMP search handling.

Rewrite of IMP search handling.

Total rewrite of search system.

Search queries, elements, and virtual folders broken into separate
classes for easier generation and better encapsulation of the data.
Also allows for more complex search types - for example, the "To:"
header search has been replaced by the Recipients search, which searches
all of To/Cc/Bcc. Another example: the bulk search query has been
created (although it is not yet implemented in the UI).

Virtual folder maintenance is now handled within a prefs groups to
consolidate duplicate code in both imp and dimp.

Internal storage of virtual folders has changed. IMP 4 virtual folders
will be automatically converted, but any virtual folders created using
git code in the last year or so is now invalid and no upgrade path is
available.

Removed the use_vinbox, use_vtrash, vinbox_id, and vtrash_id prefs.