Remove setting: `focus_by_distance`

The removal is motivated by the following example:

+-------+-------+
|       |       |
|       |   b   |
|       |       |
|       |_______|
|   a   |       |
|   *   |   c   |
|       |       |
|       |_______|
|       |       |
|       |   d   |
|       |       |
+-------+-------+

Bellow is a summary of the number of steps required to reach *b*, *c* and *d* with *a* as a starting point with the associated mean and variance:

focus_by_distance         steps           mean         variance
---------------------------------------------------------------
  false                   1, 2, 3           2            2/3
  true                    2, 1, 2          5/3           2/9
Merge pull request #36 from Stebalien/fix-monitor-selection

Call `monitor_from_desc` in `monitor -f <mon>`
Apply modifiers to all objects

Window:
 * cycle                - no change
 * direction            - only consider matching nodes
 * last                 - walk history
 * focused              - match or NULL
 * biggest              - find biggest matching

Desktop:
 * cycle                - no change
 * focused              - match or NULL
 * last                 - match or NULL

Monitor:
 * cycle                - no change
 * direction            - only consider matching nodes
 * last                 - match or NULL
 * focused              - match or NULL