Recency heuristic for ambiguous output selection

[ccjmne]

Discussed in: #2897

This Pull Request changes the algorithm to determine which output to target when going up, down, left or right: it implements some MRU heuristic for all the commands that have to do with choosing an output up, down, left or right.

I also considered making it opt-in, with, say:

Mod+Shift+Left  { focus-monitor-left  use-focus-history=true; }

But there are so many such functions; at least all of these, in 4 directions:

Mod+Ctrl+Down          { move-window-down-or-to-workspace-down; }
Mod+Ctrl+WheelScrollUp { move-column-to-workspace-up;           }
Mod+Shift+Ctrl+Left    { move-window-to-monitor-left;           }
Mod+Shift+Left         { focus-monitor-left                     }
Mod+Up                 { focus-window-or-workspace-up;          }

And I figured people would generally want to have a single selection scheme for all their output selection commands. In any case, we can discuss this and think about it some more, but I think there's something pretty usable here as is it.

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Just like before, the candidate outputs are:

• in the correct direction, and
• they are at least partially in the (virtually) infinite "strip"
  obtained from extending the active output either vertically, for up
  and down motions, or horizontally for horizontal movements.

Unlike before, we now rank the candidate outputs by their nearbymost
vertical (or horizontal) edge: this allows us to more reasonably end up
with ties, where the ambiguity that is natural to the human user also
becomes effective in the code.

We curate a stack of the most recently used monitors and use it to
resolve the ties against the candidates that share the closest edge to
the currently active output.

If none of the candidates were ever visited, we fall back to the
previous heuristic, which would find the closest candidate, according
to the distance between their geometric centre and that of the active
output.