keyboard: set keymap on content pane, not root pane.

This way, when the menu bar is active, the normal menu key sequences
(such as ESCAPE) will still work normally.

One quirk is the following-
* MINUS will still zoom out while the menu is active,
but PLUS will not work.

git-svn-id: https://micropolis.googlecode.com/svn/trunk/micropolis-java@778 d9718cc8-9f43-0410-858b-315f434eb58c