- Adds docker support for building Windows binaries on Linux.
- Update libuv to version 1.44.1.
- Use MinGW compiled 64bit Windows binaries for distribution.
In order to use the gdnative library as an editor plugin it was
neccessary to set the `reloadable` property of the gdnlib file to false,
in order to prevent crashes when the godot editor window lost focus.
This may have consequences when recompiling the library.
See: https://docs.godotengine.org/en/3.3/classes/class_gdnativelibrary.html#class-gdnativelibrary-property-reloadable
Still crashes quite frequently and doesn't close child processes
properly.
Part of #43.
