New users find Minetest's account system confusing.
This change moves username/password to a new dialog,
with login and register buttons added to the Join Game tab.
The old registration confirmation dialog is removed in
favour of the new dialog.
Fixes#8138
- Add warning log level
- Change debug_log_level setting to enumeration string
- Map Irrlicht log events to MT log events
- Encapsulate log_* functions and global variables into a class, Logger
- Unify dstream with standard logging mechanism
- Unify core.debug() with standard core.log() script API
