Localization for devtools-html : retiring DTDs and promoting LocalizationHelper

[Julian Descottes]

If you are never changing/adding/reviewing localized strings in devtools
you can stop reading now.

For devtools-html we are making 2 main changes to our localization:
- localized strings will all be in *.properties files. No more DTDs
- localization should rely on l10n.js::LocalizationHelper instead of
Services.string

I am filing bugs for our modules to make them compliant with the 2 rules
above. In priority we target the inspector, the console and the toolbox.

=> Until the migration is complete:
- avoid adding new strings in DTD files
- if you are about to create a new wrapper/helper on top of Services.string,
consider using LocalizationHelper directly instead

Thanks!
Julian

(check https://bugzilla.mozilla.org/show_bug.cgi?id=1290947 and all the
blocking bugs for more information)