BUG: Don't overwrite entries in the KEY_MAP dictionary.

This corrects a regression introduced by #121.

Basically, the key names defined in `enable.toolkit_constants` need to be used by both Qt and WX. By generating its own key names, the `KEY_MAP` for the Qt backend became subtly incompatible with WX.

This was found when some existing code which uses `KeySpec('Esc')` stopped working after pulling the changes introduced by #121 (thanks @noraderam!).

To solve this problem, I've added an extra check in the loop which fills in the `KEY_MAP` dictionary so that keys which were already added (using their proper `toolkit_constants` names) won't be overwritten.