Move translate_keysyms to bindings.[ch]

Additionally add a check so the function only handles bindings of type
B_KEYBOARD to prepare for the new bindmouse feature.

include/config.h

@@ -308,12 +308,6 @@ struct Barconfig {
  */
 void load_configuration(xcb_connection_t *conn, const char *override_configfile, bool reload);
 
-/**
- * Translates keysymbols to keycodes for all bindings which use keysyms.
- *
- */
-void translate_keysyms(void);
-
 /**
  * Ungrabs all keys, to be called before re-grabbing the keys because of a
  * mapping_notify event or a configuration file reload