Runtime gettext for Corex components.

Host applications configure config :phoenix, :gettext_backend, MyApp.Gettext so gettext/1 and gettext/2 resolve to the app backend at run time.

Do not call bare gettext(...) inside attr(..., default: ...) on components: that runs while the module compiles and can bind to the host backend before it exists when :corex is a path dependency. Use a literal default, then call Corex.Gettext.gettext/1 at the start of the render function (see Corex.Select and Corex.Combobox).

In component modules that use Phoenix.Component, prefer Corex.Gettext.gettext/1 and Corex.Gettext.gettext/2 instead of import Corex.Gettext + gettext, so calls are never confused with Phoenix or Gettext macros.