Do not repeatedly press Enter until the GNOME Run dialog vanishes.

The repeated pressing was introduced earlier (in commit c80ae9e) to
workaround a possible GTK bug (or similar). However, if sikuli is
faster than GNOME, sikuli may `find` the GNOME Run dialog even after
pressing Enter, hence pressing Enter more than once. The extra Enter:s
may propagate to whatever application that was started by the run
dialog, which in the case of the Unsafe Browser, for instance, will
close its verification window.

To workaround the original issue we instead simply sleep for a short
time inbetween typing the command and pressing Enter.