MailFilterAgent: implement support for 'All folders' filters

When a filter has the 'Apply to inbound emails in all folders' option enabled,
the MFA will monitor all Collections and will match the filter against all
incoming emails in all folders. This option combines with the 'Only apply to
account' option, so one can restrict the filter to only apply against all
folders from only a specific account.

Differential Revision: https://phabricator.kde.org/D7135