Automatic merge of T1.5.1-925-g1c557094e and 18 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 3ca0eb1: Blueprint/train car operations UI window
- Pull request #885 at 0a956f5: 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 #903 at 7af1f91: Downloading route content (Github, zip)
- Pull request #912 at f7b85e4: New Triple Valve Features Vol. 2
- Pull request #919 at 26cc6a8: Added mouse wheel support for controls which can be moved by pressing t…
- Pull request #923 at cc1b5b6: Add curve squeal to route
- Pull request #924 at 6c2c3cd: Default Asset Improvements
- Pull request #925 at e3b1688: Fix brakeshoe force bug
- Pull request #927 at 7bce293: AI Train Power Supply/Lights Fix
- Pull request #929 at a406a69: Add support for common circuit breaker variants
- Pull request #930 at 42398f4: Further enhancements for duplex locoomotive
- Pull request #932 at 9bdb074: Changed matrix's to matrices

Author: openrails-bot

Source/ORTS.Common/Coordinates.cs

@@ -21,8 +21,8 @@
  * whereas in MSTS it is the opposite.  As a result you will see the sign of all Z coordinates gets negated
  * and matrices are adjusted as they are loaded into XNA.  In addition the winding order of triangles is reversed in XNA.
  * Generally - X,Y,Z coordinates, vectors, quaternions, and angles will be expressed using MSTS coordinates 
- * unless otherwise noted with the prefix XNA.  Matrix's are usually constructed using XNA coordinates so they can be 
- * used directly in XNA draw routines.  So most matrix's will have XNA prepended to their name.
+ * unless otherwise noted with the prefix XNA.  Matrices are usually constructed using XNA coordinates so they can be 
+ * used directly in XNA draw routines.  So most matrices will have XNA prepended to their name.
  * 
  * WorldCoordinates
  * X increases to the east