Always allow translation by the Cinnamon domain

Right now the translation in settings need to be done or by Cinnamon domain or by the desklet and applet domain, this can be improve to allow always the cinnamon domain translation, checking if not exist a traslation in the applet and desklet domain for specified key, this can be done asking if key == traslation in the applet and desklet domain, in this case the key can be send to the cinnamon domain to be translated... This allow to the developers of applet and desklet not need repeat a translation that Cinnamon have. This mechanims also can be implemented in the .js files with a little changed in the normal implementation of the function:

function _(str) {
return Gettext.dgettext(uuid, str);
}

by this one

function _(str) {
let resultUUID= Gettext.dgettext(uuid, str);
if(resultUUID != str) {
return resultUUID;
}
return Gettext.gettext(str);
}

The changed that we need to do, for implemented this are minimal and the effect can be excelent...
Update XletSettingsWidgets.py
Update XletSettingsWidgets.py
Update XletSettingsWidgets.py
Fixed the Accidental delete

Fixed the Accidental delete of the latest code of @mtwebster
Optimization: make less validations
Merge pull request #2814 from lestcape/master

Always allow translation by the Cinnamon domain