libobs: Add config_has_(user|default)_value functions

Similar to obs_data_has_(user_value|default|autoselect), these allow
library users to distinguish between user config values and default
values
Use macro for setting the config default language
Add helper function for locale (name) enumeration
Use locale (name) enumeration helper in basic settings window
Add platform specific locale priority enumeration

These functions match the known obs locales with the system supplied
locales and return a vector of possible locales with the highest
priority locale first
Implement language autoselection using platform priority lists
Show current locale in basic settings dialog