LogCombiner GUI should pick up burn-in more robustly

[rbouckaert]

When changing the burnin in the GUI a return key is required for the value to be accepted.
When no return key is pressed, switching focus to another burnin value resets the just edited burnin value.
However, focusing outside the list of log files does not cause the value to change, but the value entered is not accepted.

Values entered in the burnin entry should be processed after releasing a key.