Gerrit: Do not run plink executable in accessor of OptionsWidget.

Do not run the process every time the user presses 'Ok' or 'Apply'
in the settings page, do it only if the gerrit parameters actually
changed. Also check for empty binary.

Change-Id: I934616e5d9b20404278f1fd9fc751ff161030cd8
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>