Arena Levels

Added two arena levels with a scoring system. The levels are accessed from Mission Select -> Extra Levels.

Levels
- Arena: Factory: straightforward arena without gimmicks.
- Arena: Train Mine: arena with 4 deadly trains speeding through regularly.
- The levels have 40 waves, with 8 waves based on each of the 5 environments.

Scoring
Tech point from enemies is base score.
Bonuses: (these stack multiplicatively)
- Wave clear speed: up to +100% (falls gradually to 0% over 30 seconds)
- No damage taken: +20%
- Difficulty: normal +20%, hard +50%, insane +100%

Pickups
- Enemies will drop only ammo pickups (so the arena won't get filled with shield pickups floating in the air if you don't take damage)
- There are some shield pickups initially. Shield, ammo, and power-up pickups are also spawned after specific waves. 

Other
- Music changes after specific waves.

UI

Added a sound and an additional UI effect when player gets locked on by enemy homing missiles.

Quick help section in pause menu updated.

Bugfixes

Projectiles which ricochet from walls no longer disappear when hitting a door. Affects Chain Lightning weapon and Bouncer enemies.

Fixed bug where Mission Select menu allowed skipping missions in some case.

Beaming: Fixed issues where encounters could stop progressing.