Automatic merge of T1.5.1-770-g1f196a324 and 15 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 #875 at 43bf33e: Bug fix for https://bugs.launchpad.net/or/+bug/2036346 Player train switching doesn't work with 3D cabs
- 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 d9ce84b: 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 #893 at bf8876b: Signal errors
- Pull request #894 at 5ff1e73: Correct Decrease Colour
- 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 #898 at 2d1b44a: Extra line with all the arguments for debugging purposes in logfile
- Pull request #899 at c17fdb3: Duplex steam engines - Booster Engine addition
- Pull request #903 at a0d2991: first phase downloading from Github