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.
Bug 1062807 - Use the new shared helper for loading JSON files in the wallpaper app.
Merge pull request #24216 from lissyx/bug1068860

Bug 1068860 - Allow ModalDialog to display pure strings r=gandalf
Bug 1069415 - Since bug 1002545 landed, we can't commit CSS files under Linux r=yurenju
Merge pull request #24115 from ddrmanxbxfr/bug_1062807

Bug 1062807 - Use the new shared helper for loading JSON files in the wallpaper app r=@yurenju