feat(config): add the trash query generator

Adds trash as a fifth built-in query filter beside Unread, Inbox,
Important and Sent, composing per-account exactly as Sent does:
Config::resolvedQuery() asks each account for its own trashQuery()
rather than wrapping the all-accounts union, and an account with no
trash folder resolves to matchNothingQuery() rather than "match
everything".

Also gives the Trash button a toolbar icon (user-trash) and a trash
key to the mainwindow fixture that asserts every filter button carries
one; without it the button is skipped from the row entirely (no
account configured a trash folder), and the existing icon test found
no button to check.

0 files changed, 0 insertions, 0 deletions