dot/emacs, el/dot-emacs.el: New gadget to force window selection.

I'm fed up with Emacs `display-buffer', which seems carefully designed
to read my mind and choose the least convenient possible window display
a buffer in.

Introduce a new command `mdw-designate-window', bound to `C-c w SPC',
which marks a window as being the target for the next buffer shown by
`display-buffer' when its caller doesn't have a better plan.  This
leaves alone things like Magit popups, which already know that they want
to go at the bottom of the current window (and that's fine).