Merge gaps support as-is

This code was copied over unmodified from https://github.com/Airblader/i3-gaps.

I have split out the differences between i3-gaps and i3 into three areas:
1. Gaps
2. i3bar height
3. rgba colors

include/con.h

@@ -525,6 +525,13 @@ void con_set_urgency(Con *con, bool urgent);
  */
 char *con_get_tree_representation(Con *con);
 
+/**
+  * Calculates the effective gap sizes for a container depending
+  * on whether it is adjacent to the edge of the screen or another
+  * container.
+  */
+gaps_t calculate_effective_gaps(Con *con);
+
 /**
  * force parent split containers to be redrawn
  *