V4.5.4 Update : Gravity system, viewport support, preview patterns,...

Hillow ! This is the last content update for a while.

All the bug reports have been patched.

New features :

1. Basic gravity system for 2D sidecam games like plateformers. Add weight to different bullets, add an initial impulse to patterns, customise the gravity forece and direction. It also allows for areas with gravity fields with different directions.
2. Preview bullet patterns and trajectories. Doesn't work for every case but it's a good help.
3. Support for having your game inside a viewport, like for Touhou-like games.
4. When spawning bullets with a position-rotation dictionnary, you can set a new "parent" key, with which set a node as the parent of the bullets.