Automatic merge of T1.5.1-794-g4cad06780 and 14 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 a055bca: Blueprint/train car operations UI window
- Pull request #885 at 06b8a9e: 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 0a9d939: feat: Improved system information collection
- Pull request #899 at 89d8d80: Duplex steam engines - Booster Engine addition
- Pull request #903 at 7353625: Downloading route content (Github, zip)
- Pull request #906 at 5850660: Bug fix for https://bugs.launchpad.net/or/+bug/2047299 Crash loading a 3Dcab-only loco
- 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 4b4afe3: feat: supports switching adhesion precisions

Author: openrails-bot