Level configuration structure rework

[ironkayman]

Current implimentation is not formalised neither structurally self-explanatory

Do not retain compatibility with existing levels, mark them as level-name.ignore and migrate later

• Merge Level scene and Invasion view, minimise utilisation of Wave scene, delegating to it only updates and basic sprite management of it's registered spawners, Invasion view renamed to Mission
• Rework alien state texture storage, to dict, and later to Atlas
• Migrate Mission view gameplay on-update functions, simplify them, remove excess random module calls
• Fix GameOver scene and Mission's intermediate state, graceful exit to MainMenu view
• Fix Mission's waves' change and completion
• Alien approach speed should remain configurable through wave configuration