0.4.0 Release: New Game Engine, Map, Upgrades

After a shockingly long development time of roughly seven months, update 0.4.0 is finally ready.

Calling this a mere update is a bit of an understatement. During the last four of those seven months, I've been rebuilding the game in a different engine. Everything had to be re-coded, and all resources had to be reformatted.

That may seem extreme, but the results are worth it: Despite being greatly expanded in scope, the new version runs faster, uses less memory, and its file size is only about half of the previous version. In short, it's no longer handicapped by a monstrously bloated engine. Additionally, the project files are less than 5% of their original size. Yes, the game is now a tiny fraction of what it used to be, despite being more feature-complete and having much more content. Now that's efficiency; something a lot of developers don't seem to understand or care about.

Aside from the completely remade gameplay systems and various improvements, there have been a few major additions.
- A new jet model. Fun fact: The old one was designed as a spaceplane, which is why it looked so unusual. It was a relic of the game's earliest plans, which would have involved alien worlds and spaceflight.
- A world map, with random hostile encounters.
- Upgrades for the cannon, engines, and hull.
- A short three-mission campaign, which gradually unlocks as you win hostile encounters.

This update also adds Linux builds. They're completely untested, but they should work... probably.

Although I've fixed all the game-breaking issues I could find, I fully expect this update to be incredibly unstable. To prepare for the inevitable bug reports, I've made a Discord server for quick feedback.
https://discord.gg/K568kTfCBh

Semi-complete list of changes:
- Ported to the Godot Engine. Rebuilt + optimized game functions from scratch
- Overhauled main menu: Intro screen, new background, cinematic style
- Improved bullet firing sound
- Improved main menu music
- Revised GUI style
- Redesigned player jet
- Added landing gear
- Redesigned game logo
- Added more video options: Resolution, shadows, post processing, FPS limit
- Removed energy bar
- Removed Obstacle Course mode
- Added world map with random enemy encounters
- Improved explosion sound and visuals
- Improved turret model
- Improved missile model and icon
- Missiles don't circle missed targets anymore
- Added HUD damage effect
- Added more paint options
- Added cannon, engine, and hull upgrades to the workshop
- Added more paint options to the workshop
- Added a basic three-mission campaign, which can be unlocked by defeating
  random hostile encounters