Impliment i3-style marks

This commit adds three commands to sway: `show_marks`, `mark` and
`unmark`. Marks are displayed right-aligned in the window border as i3
does. Marks may be found using criteria.

Fixes #1007

include/sway/config.h

@@ -275,6 +275,7 @@ struct sway_config {
 	bool reading;
 	bool auto_back_and_forth;
 	bool seamless_mouse;
+	bool show_marks;
 
 	bool edge_gaps;
 	bool smart_gaps;