Fixes and Esc to pause

Hopefully this fixes the crash some of you have been having when a level ends, either just before the gameover screen appears or on the gameover screen itself. Let me know. I've also enabled debug logging. Logfiles are stored on your computer and do not contain any personal information. Logfiles are not transmitted to me or stored on the internet. They are private. I've enabled them to help me troubleshoot problems you may be having. If you have a crash, the logfile may have some clues about what happened. Here's what a typical file looks like:

OpenGL ES 3.0 Renderer: GeForce GTX 1060 6GB/PCIe/SSE2
Loading demo 3
**ERROR**: Condition ' _first != __null ' is true.
   At: core/self_list.h:100:~List() - Condition ' _first != __null ' is true.
**ERROR**: Condition ' _first != __null ' is true.
   At: core/self_list.h:100:~List() - Condition ' _first != __null ' is true.

By popular demand, the Esc key now pauses the game. And destroyed flying drones are no longer affected by gravity. They just float away forever into the void of space. So sad.

• (possible) Fix for crash on gameover screen (fingers crossed)
• Enabled debug logging. Logfiles contain debug and crash info and are located: Mac/Linux: ~/.local/share/godot/app_userdata/Gravity Windows: C:\users\USER\AppData\Roaming\Godot\app_userdata\Gravity
• Esc key now pauses the game when playing. In the editor, Esc exits the editor.
• Destroyed flying drones are no longer affected by gravity