Add API for setting/getting current locale

This API is used to set the current locale for libobs, which it will set
for all modules when a module is loaded or specifically when the locale
is manually changed.
Remove 'locale' from properties

Having the value stored here is somewhat pointless, so this is one step
in fixing the locale handling.  Locale should be handled by the modules
themselves with their own loaded locale lookup information.
Remove 'locale' parameter from all callbacks

The locale parameter was a mistake, because it puts extra needless
burden upon the module developer to have to handle this variable for
each and every single callback function.  The parameter is being removed
in favor of a single centralized module callback function that
specifically updates locale information for a module only when needed.
Add module callbacks for loading locale data

The module callback obs_module_set_locale will be called after loading
the module, and any time the locale is manually changed via core API.

When this function is called, the module is expected to load new text
lookup values for all the text it uses based upon the current locale.