Automatic merge of T1.5.1-866-gbae4730e2 and 17 pull requests

- Pull request #570 at 3539862: Experimental glTF 2.0 support with PBR lighting
- Pull request #839 at d00beb9: First phase of https://blueprints.launchpad.net/or/+spec/additional-cruise-control-parameters
- Pull request #876 at f92de76: docs: add source for documents previously on website to source Documentation folder
- Pull request #882 at 9c456aa: Blueprint/train car operations UI window
- Pull request #885 at 8f94333: feat: Add notifications to Menu
- Pull request #886 at 6c0785b: Scene viewer extension to TrackViewer
- Pull request #892 at 1f5ba4c: Signal Function OPP_SIG_ID_TRAINPATH
- Pull request #896 at 5866028: First implementation of https://blueprints.launchpad.net/or/+spec/specific-sounds-for-ai-trains
- Pull request #897 at 42f1dd9: feat: Improved system information collection
- Pull request #903 at 0d6d045: Downloading route content (Github, zip)
- Pull request #907 at 9b0b04f: Bug fix for https://bugs.launchpad.net/or/+bug/2047300 Dynamic tracks disappear after long tunnel
- Pull request #908 at ad3362c: feat: supports switching adhesion precisions
- Pull request #911 at 6834af0: docs: Add refactoring as a special type of PR
- Pull request #912 at 659396e: New Triple Valve Features Vol. 2
- Pull request #914 at 69a4ccc: Adjustments to Duplex steam
- Pull request #915 at 6d911d7: Correct calculation error with curve friction
- Pull request #916 at 11ac52c: Distributed Power Air Brake Synchronization

Author: openrails-bot