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/render.h

@@ -40,7 +40,7 @@ typedef struct render_params {
  * updated in X11.
  *
  */
-void render_con(Con *con);
+void render_con(Con *con, bool already_inset);
 
 /**
  * Returns the height for the decorations