Fix issue in running compare from the command line

Fix issue in running compare from the command line

Because Compare uses threads, we need to make sure it is finished when a
compare is executed from the command line. This was a problem for command
sequences, because the next command would start running before the compare
was done, and the buffer with differences was available.

So this commit adds the "run-blocking" parameter when using
LFUN_DIALOG_SHOW to run a Compare. When calling Compare with run-sync, the
LFUN will wait for the compare worker thread to finish before returning and
possibly running the next command.