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/commands.h

@@ -115,10 +115,10 @@ void cmd_workspace_back_and_forth(I3_CMD);
 void cmd_workspace_name(I3_CMD, const char *name);
 
 /**
- * Implementation of 'mark [--toggle] <mark>'
+ * Implementation of 'mark [--add|--replace] [--toggle] <mark>'
  *
  */
-void cmd_mark(I3_CMD, const char *mark, const char *toggle);
+void cmd_mark(I3_CMD, const char *mark, const char *mode, const char *toggle);
 
 /**
  * Implementation of 'unmark [mark]'

include/con.h

@@ -158,13 +158,13 @@ bool con_has_mark(Con *con, const char *mark);
  * Otherwise, the mark is assigned to the container.
  *
  */
-void con_mark_toggle(Con *con, const char *mark);
+void con_mark_toggle(Con *con, const char *mark, mark_mode_t mode);
 
 /**
  * Assigns a mark to the container.
  *
  */
-void con_mark(Con *con, const char *mark);
+void con_mark(Con *con, const char *mark, mark_mode_t mode);
 
 /**
  * If mark is NULL, this removes all existing marks.

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.
  */