Allow multiple marks on windows.

This patch allows multiple marks to be set on a single window. The restriction that a mark may
only be on one window at a time is still upheld as this is necessary for commands like
"move window to mark" to make sense.

relates to #2014

include/con.h

@@ -146,6 +146,12 @@ Con *con_by_frame_id(xcb_window_t frame);
  */
 Con *con_by_mark(const char *mark);
 
+/**
+ * Returns true if and only if the given containers holds the mark.
+ *
+ */
+bool con_has_mark(Con *con, const char *mark);
+
 /**
  * Toggles the mark on a container.
  * If the container already has this mark, the mark is removed.