Linux: freedesktop support for file type association

Support freedesktop file association on Linux/Unix via the command line
arguments: `--register{-allusers}` `--unregister{-allusers}` as well
registration actions from the user preferences.

Once registered, the "Blender" application is available from launchers
and `*.blend` files are assoisated with the blender binary used for
registration.

The following operations are performed:

- Setup the desktop file.
- Setup the file association & make it default.
- Copy the icon.
- Setup the thumbnailer (`--register-allusers` only).

Notes:

- Registering/unregistering for all users manipulates files under
  `/usr/local` and requires running Blender as root.
  From the command line this can be done using `sudo`, e.g.
  `sudo ./blender --register-allusers`.
  From the GUI, the `pkexec` command is used.

- Recent versions of GNOME execute the thumbnailer in a restricted
  environment (`bwrap`) requiring `blender-thumbnailer` to be copied
  into `/usr/local/bin` (synlinks don't work).
  So thumbnailing copies the binary rather than linking and only works
  when registering for all users.

Ref !120283

scripts/startup/bl_ui/space_userpref.py

@@ -667,15 +667,22 @@ class USERPREF_PT_system_os_settings(SystemPanel, CenterAlignMixIn, Panel):
 
     @classmethod
     def poll(cls, _context):
-        # Only for Windows so far
-        import sys
-        return sys.platform[:3] == "win"
+        # macOS isn't supported.
+        from sys import platform
+        if platform == "darwin":
+            return False
+        return True
 
     def draw_centered(self, context, layout):
-        if context.preferences.system.is_microsoft_store_install:
-            layout.label(text="Microsoft Store installation")
-            layout.label(text="Use Windows 'Default Apps' to associate with blend files")
-        else:
+        from sys import platform
+        associate_supported = True
+        if platform[:3] == "win":
+            if context.preferences.system.is_microsoft_store_install:
+                layout.label(text="Microsoft Store installation")
+                layout.label(text="Use Windows 'Default Apps' to associate with blend files")
+                associate_supported = False
+
+        if associate_supported:
             layout.label(text="Open blend files with this Blender version")
             split = layout.split(factor=0.5)
             split.alignment = 'LEFT'