Introduce "--add" for marking windows.

In order to keep compatibility to before allowing multiple marks on a window,
we introduce a flag "--add" that must be set to put more than one mark on a
window. The default, which is also available as "--replace", keeps the old
behavior of overwriting a mark when setting a new one.

fixes #2014

include/data.h

@@ -75,6 +75,9 @@ typedef enum { ADJ_NONE = 0,
                ADJ_UPPER_SCREEN_EDGE = (1 << 2),
                ADJ_LOWER_SCREEN_EDGE = (1 << 4) } adjacent_t;
 
+typedef enum { MM_REPLACE,
+               MM_ADD } mark_mode_t;
+
 /**
  * Container layouts. See Con::layout.
  */