Make fullscreen windows open on the output which is indicated by their geometry

With this change, multi-monitor presentations (e.g. as implemented by
LibreOffice Impress) work out of the box. Previously, one had to move
the presentation windows to the right outputs oneself.

include/con.h

@@ -268,6 +268,13 @@ void con_disable_fullscreen(Con *con);
 void con_move_to_workspace(Con *con, Con *workspace, bool fix_coordinates,
                            bool dont_warp, bool ignore_focus);
 
+/**
+ * Moves the given container to the currently focused container on the
+ * visible workspace on the given output.
+ *
+ */
+void con_move_to_output(Con *con, Output *output);
+
 /**
  * Moves the given container to the given mark.
  *