Make it easier to call localization functions

[ibrahim-mubarak]

We need to do the following

• Expose global tr and pl functions for translate and pluralize.
• Don't require context for calling Localization functions.

To do away with needing BuildContext for localization, we should set the BuildContext through the page mixin. We also need to create a mixin which should be used when displaying static content.

[ibrahim-mubarak]

This is no longer relevant with the the I18n Json Plugin. We should integrate this in our framework though.