Add folder-level 'Apply filter' actions to KMail

This adds 'Apply Filters on Folder' and 'Apply Filters on Folder and
its Subfolders' actions to KMail main menu. This allows to simply
execute any filter (or all filters) on an entire folder with a single
click.

This change also extends the Mail Filter Agent DBus API to allow callers
to pass Collection IDs to the Agent and the Agent will resolve the Items
itself, instead of the caller having to first obtain the Items and then
pass all of them via DBus.