Reset stacking client list whenever appropriate
Allow selecting window for bspc actions

Instead of always acting on the focused window, allow setting the
selected window by prefixing the command with
`--window-id my_window_id`. This allows better scripting of window
actions.

It also, by necesity, changes a few commands:
 1. `focus` can now be called with no arguments to focus the selected
    window.
 2. `drop_to` can now be called with `here` in addition to `next` and
    `prev` to allow dropping a window to the current desktop.
Take either an int or hex id.