Feature: user_desktop_dir

[tepiloxtl]

In similar vain to other user library locations, I'd like to propose/add user_desktop_dir:

• Windows: CSIDL_DESKTOP - e.g. %USERPROFILE%\Desktop (Windows does provide both CSIDL_DESKTOP, which combines user Desktop dir and common Desktop dir, and CSIDL_DESKTOPDIRECTORY which provides only "physical" user Desktop directory, I went with more complete option here)
• Unix: XDG_DESKTOP_DIR - e.g. $HOME/Desktop
• MacOS: os.path.expanduser("~/Desktop")
• Android: I think it's irrelevant on Android, but for sake of compatibility some directory should be provided, I'm open to suggestions

I'm willing to make a PR with those changes

[gaborbernat]

Please create the PR for this.

[gaborbernat]

PR opened under https://github.com/platformdirs/platformdirs/pull/200