Event routing

[yatli]

Consider the following situation:

 MC1 <--- MC2
 | |      |
MD A4     MNM

For MC1, I'd like to use A4 keys in step editing.
For MC2, I'd like to use A4 keys in step editing.

We'll have to make the pages actively acquire and release the A4 keyboard as a resource, and route the messages accordingly.