Debugger: fix race condition in block mode

Make sure that the GUI thread & debugger thread actually sync on startup. So far
the GUI thread would block forever in waitCondition.wait() if the debugger thread
spawns & receives the HELLO before.

Also remove unused code and rename variables to make their use more obvious.

Change-Id: I8285e8860667496d491807e696535353d9f14dea
Reviewed-by: Aurindam Jana <aurindam.jana@digia.com>