Remove old and unused project settings, update various demos for 4.2 (#1024)

- Move all demo projects that don't require Forward+/Mobile-only features
  to the Compatibility rendering method. This improves performance significantly
  on low-end devices and ensures visuals are identical to a web export
  of the demo.
- Set deadzone on all inputs to 0.2 for better gamepad usability.
- Remove reliance on `default_env.tres` to use built-in Environment
  resources in the main scene instead (which follows the preview environment
  workflow).
- Remove notices pointing to GDNative or VisualScript, since both were
  removed in 4.0.
- Various bug fixes and usability tweaks to 10+ demos.

gui/input_mapping/ActionRemapButton.tscn

@@ -6,4 +6,4 @@
 offset_right = 90.0
 offset_bottom = 30.0
 toggle_mode = true
-script = ExtResource( "1" )
+script = ExtResource("1")