Allow translations of applet, desklet, and extension settings via gettext/po/mo files

This change allows applet (or desklet or extension) settings to pull translations from applet-supplied translation files for description, tooltip, units value fields, as well as combo/radio key fields.

It begins by searching for a translation domain of <applet uuid> in the user locales, then system locales, and finally falls back to the cinnamon domain (which will allow us to translate system applet settings).

This should sidestep javascript issues like #2245 with regards to UTF-8 mangling of strings.

Minor testing done using the weather applet.  More can be done once I create the extraction utility.