New formspec elements:
- `scroll_container[<X>,<Y>;<W>,<H>;<scrollbar name>;<orientation>;<scroll factor>]`
- `scroll_container_end[]`
Other elements can be embedded in this element. Scrollbar must be placed manually.
* Attempt to add registration confirmation
Using SRP auth mechanism, if server sent AUTH_MECHANISM_FIRST_SRP that means the player isn't exist.
Also tell player about the server and chosen username.
Local game has localhost as IP address of the server.
Add RenderingEngine::draw_menu_scene() to draw GUI and clouds background.
aborted -> connection_aborted
* Rewrite information message text
Client::promptConfirmRegister() -> Client::promptConfirmRegistration()
