Include string for about dialog

With #300 being a feature scheduled for 0.2.0, we should make sure to
inlcude those strings before the string freeze on Feb 15.

Once this is merged we should add a comment on Transifex to give
translators some context.

src/main/resources/strings/BriarDesktop.properties

@@ -130,6 +130,7 @@ welcome.text=You don't have any contacts yet. Tap the + icon to add a contact:
 about.copyright=Copyright
 about.license=License
 about.version=Version
+about.version.core=Core Version
 about.contact=Contact
 about.website=Website