Add difficulty levels to New Game screen
Merge remote-tracking branch 'origin/develop' into develop

Conflicts:
	Assets/Scripts/BossShootInterval.cs.meta
	Assets/Scripts/boss_scripts/Boss1_moveV2.cs.meta
	Assets/Scripts/boss_scripts/BossProjectile_V1.cs
	Assets/Scripts/boss_scripts/BossProjectile_V1.cs.meta
	Assets/Textures/buttons.meta