Add `input_type` enum to `Binding` typedef

Long description:

An input type of B_KEYBOARD will indicated this binding was created with
"bindsym", "bindcode", or "bind" and should only run on key press
events.

An input type of B_MOUSE will indicate this binding was created with
"bindmouse" and should only run on button press events (not yet
implemented).

For more information see #558.

To apply this patch, use:
curl http://cr.i3wm.org/patch/372/raw.patch | git am

@@ -213,6 +213,15 @@ struct regex {

  *

  */

 struct Binding {

+    /* The type of input this binding is for. (Mouse bindings are not yet

+     * implemented. All bindings are currently assumed to be keyboard bindings.) */

+    enum {

+        /* Created with "bindsym", "bindcode", and "bind" */

+        B_KEYBOARD = 0,

+        /* Created with "bindmouse" (not yet implemented). */

+        B_MOUSE = 1,

+    } input_type;

+

     /** If true, the binding should be executed upon a KeyRelease event, not a

      * KeyPress (the default). */

     enum {