Add ability to specify available locales on the i18nProvider

[djhi]

Problem

Supporting multiple locales is cumbersome:

• configure the i18nProvider in one file
• make a custom appbar to include the LocalesMenuButton
• remember to add the title in the appbar as it is not included automatically when passing custom children
• sync the i18nProvider locales with the LocalesMenuButton options

Solution

Defines the available locales in the same file they are added (the i18nProvider), thanks to a new getLocales method.
The Appbar will now include the LocalesMenuButton automatically if multiple locales are available

[slax57]

Some small things can be improved in the docs, otherwise that's great! 😎