v0.3.6: Shop and Upgrade tweaks

Really just a few small things here.

1. Make Firing Drone shoot an Auto Cannon bullet which is then upgraded, the base values fluctuate too much if it copies the actual weapon. I can and will revisit this but the discrepancy in Firing Drone performance based on the chosen weapon was too high.
2. +2 Projectile Count upgrade is gone. +1 Projectile Count upgrade is now Legendary.
3. Do not show Engineer players the Fire Rate upgrade, since they have no weapon and it modifies nothing. (The other weapon-based upgrades, like Projectile Damage, _do_ work, since they impact your Firing Drones.)
4. Do not show Heat Ray players the Fire Rate upgrade. The Heat Ray functions as hitscan, it does not use a fire rate.
5. Shift Projectile Damage upgrade over to a multiplicative, percentage-based upgrade. Flat value is WAY too strong on Heat Ray and completely unnecessary on Rail Gun.
   
6. I found and fixed a bug where enemy drops (e.g. coins) would be on the screen at the start of a round if the player completed the shop fast enough. I don't think this was happening before? Maybe something with Godot? In any event it's good practice to ensure those objects are freed at the end of the round.
7. There is now a timer that expires every 5 seconds. When it does, if the on-screen wave is not full, it refills it. Otherwise, it spawns one enemy. This means that a player who is not killing anything will still have more enemies spawn (one every 5 seconds, to be exact).