Bug 1068860 - Allow ModalDialog to display pure strings r=gandalf

We may need to have to display strings that are not localized and for
which it does not make sense to have localization at all. This happens
for network-generated messages that we have to display to the user, on a
CDMA network for example.
Merge pull request #24216 from lissyx/bug1068860

Bug 1068860 - Allow ModalDialog to display pure strings r=gandalf