Add an easy way to test the debug_server crash window.

It's quite annoying to install the new debug_server and test with a
crashing app everytime. So add a DebugWindow app that just shows the
debug window and returns the result.

Change-Id: I7238057a508871ce3fffc493cb73126eb852d99e
Reviewed-on: https://review.haiku-os.org/c/1460
Reviewed-by: waddlesplash <waddlesplash@gmail.com>