[D11] [UI] Added Leaderboards assets required for new page in Rust Pulse/Community Hub. Marked Sceneloader as obsolete. Added headers to certain parent classes to tidy up the variables in-editor. Added translations for leaderboard headers/texts. Added leaderboards button to main menu. Renamed sheets to help with searching for them in editor. Added new bar background texture to use for buttons instead of pre-coloured variants.