Add "Edit lists" and "Edit contexts" dialogs as in Simpletask

[t7ko]

It's already easy to add lists/contexts to an entry (with auto-completion). But it's not that easy to remove existing -- you need to navigate the cursor, then select/delete. Simpletask makes it easy: you open "edit lists" dialog, and you get lists/contexts with checkboxes. Removing is super-easy, since checked items are on the top of the list.