Allow multiple assignments of workspaces to outputs

Also makes get_assigned_output work with the primary output:
    workspace X output primary
will now work.

Fixes #555.

include/workspace.h

@@ -38,6 +38,13 @@ Con *get_existing_workspace_by_name(const char *name);
  */
 Con *get_existing_workspace_by_num(int num);
 
+/**
+ * Returns true if the first output assigned to a workspace with the given
+ * workspace assignment is the same as the given output.
+ *
+ */
+bool output_triggers_assignment(Output *output, struct Workspace_Assignment *assignment);
+
 /**
  * Returns a pointer to the workspace with the given number (starting at 0),
  * creating the workspace if necessary (by allocating the necessary amount of