Debugger: Fix enable state of breakpoint command text field

There was no actual error in practice, as the handling was lumped
together with the tracepoint checkmark, which (accidentally) did the
right thing in all cases.

Change-Id: Icf12aaf0c48855b747d25a0252c8215e259c4227
Reviewed-by: Niels Weber <niels.weber@theqtcompany.com>