Turn on text input only while I am editing a text box #33

I want the game to accept text while I am editing a name and leave text-entry mode when I click elsewhere or change tabs.

The menu should follow the field I am using instead of keeping text input enabled for the whole game. This gives the window system the right signal about whether I am entering text.

The example in PR #9 enables text input only while its name field is focused. Focus changes were checked in the example and browser. This is example behavior, not an automatic rule for every Polyworld game.

The reported concern about native input methods, such as Japanese or Chinese composition, still needs a native reproduction. These focus checks do not establish that a native input-method bug is fixed.

Ordinary browser letters are tracked separately in #28.