UI: show the windowing environment in the "About" splash

Show the windowing environment on non MS-Windows/Apple systems,
since X11/WAYLAND are selected startup there was no convenient way
for users to know which back-end was being used.

Include the windowing environment in the About splash & system-info.txt
since it will be useful for handling bug reports.

This commit adds a private API call not intended for general use
as I would like to be able to remove this later and it's only needed
in the specific case of testing if Blender is using WAYLAND or X11
(which maybe be used via XWayland).

Python scripts can already inspect the system to check which windowing
environment used, the API call is mainly useful for troubleshooting.

release/scripts/modules/sys_info.py

@@ -53,6 +53,13 @@ def write_sysinfo(filepath):
             output.write("build linkflags: %s\n" % prepr(bpy.app.build_linkflags))
             output.write("build system: %s\n" % prepr(bpy.app.build_system))
 
+            # Windowing Environment (include when dynamically selectable).
+            from _bpy import _ghost_backend
+            ghost_backend = _ghost_backend()
+            if ghost_backend not in {'NONE', 'DEFAULT'}:
+                output.write("windowing environment: %s\n" % prepr(ghost_backend))
+            del _ghost_backend, ghost_backend
+
             # Python info.
             output.write(title("Python"))
             output.write("version: %s\n" % (sys.version.replace("\n", " ")))