godot 2->3 migration guide

https://github.com/dploeger/godot-migrationnotes

* themes: replace fixed fonts with .ttf (looks great!)
* CanvasItem.set_hidden(true) -> hide()
* dictionary.parse_json() -> parse_json()
* ev.type == InputEvent.KEY -> ev is InputEventKey
* ev.scanCode -> ev.get_scancode_with_modifiers() (??)
* Image(...) -> Image.new() & create(...)
* Image.fill() -> Image.fill(Color(0,0,0,0))
* Image.put_pixel -> set_pixel
* lock & unlock for Image.get_pixel & set_pixel
* set_pos() & get_pos() -> position
* Label.set_pos -> set_begin (?)
* OS.get_frames_per_second -> Engine.
* string.basename() -> get_basename()
* string.extension() -> get_extension()