[imp] DIMP + "delete" key

[Liam Hoekenga]

> If there is concern that this changes the default "go back a page"
> behavior of the backspace key, maybe we should use the browser detection
> to figure out what platform the user is using, and only add
>
>    case Event.KEY_BACKSPACE:
>
> if they user is on a mac (where the "delete" key apparently sends backspace)

Apparently, this breaks the ability to us the backspace key in the DIMP 
serach box on IE 6 (and other?) and Firefox 2 on WinXP, and Safari on OSX 
(when Safari works with DIMP)

Strangely, the delete key on my powerbook (which sends BACKSPACE) will 
both delete messages in the DIMP index pane *and* delete text in the 
search box when I use it with Firefox 2 for OSX.

Liam