Match filter and column with todo navigation

It's pretty awkward to go from the home row to the cursor keys in order to manipulate the filter view. This change adds two tiny wrapper widgets that bind j and k to move select projects and contexts and H and L to switch between todo and filter list.

In general, it would make sense to make these things more customizable in the long run.

P.S.: According to the urwid docs, overriding the command map dict is not a hack and the way to go.