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/container.h

@@ -165,6 +165,11 @@ struct sway_container {
 	 * Number of slave groups (e.g. columns) in auto layouts.
 	 */
 	size_t nb_slave_groups;
+
+	/**
+	 * Marks applied to the container, list_t of char*.
+	 */
+	list_t *marks;
 };
 
 enum visibility_mask {