Complete unfinished UI overhaul

Background

Because the Client scene has been split into initialising, main menu, and world scenes, a new UI has to be created to serve all scenes. Some screens, such as the server select screen have been removed in the migration and needs to be readded.

Proposal

Create a new graphical user interface using the new UI toolkit. Update existing ones to work with the new scene partitioning. More in debth on the website.

• Migrate error and message boxes.
• Migrate busy loading.
• Migrate preloading screen.
• Rework create character menu.
• New server select screen.
• Tweak animations and buttons to look correct.
• Add support for hotkeys.

Migrating settings

• Works both on the main menu and ingame.
• Explicitly save when state is changed.
• Create one new settings class for all settings.
• Rework controls settings (currently disabled).

Start with the Client scene partitioning and the UI overview linked in the issue, then review the existing migrated screens and remaining checklist items. Done means the create-character and server-select screens are restored, animations, buttons, and hotkeys work, and settings operate in both main-menu and in-game contexts with explicit saves and reworked controls.