platformdirs: introduce user_pictures_dir()

[kemzeb]

Introduces the means to acquire the user's pictures directory path.

Feature suggested in #141.

I have included tests (thanks to user_documents_dir much of the code including testing code was borrowed). I did test this on Windows and MacOS and from calling this function they do produce expected results. However I'm not familiar with Android with the exception of some of the developer web pages I explored that allowed me to find the DIRECTORY_PICTURES constant, and I also don't have access to an Android device so I have not tested it in a similar manner as Windows and MacOS.

The following are just references to the code changes I made that I thought were worth mentioning:
Android:

• Environment.DIRECTORY_PICTURES: https://developer.android.com/reference/android/os/Environment#DIRECTORY_PICTURES
• /storage/emulated/0/Pictures (found this path across different problems discussed in this site): https://android.stackexchange.com/questions/102962/with-the-stock-gallery-app-how-can-i-open-an-arbitrary-image-file-by-specifying

Unix:

• XDG_PICTURES_DIR : https://wiki.archlinux.org/title/XDG_user_directories

Windows:

• CSIDL_MYPICTURES
  • https://learn.microsoft.com/en-us/windows/win32/shell/csidl
  • https://github.com/Alexpux/mingw-w64/blob/master/mingw-w64-headers/include/shlobj.h#L28
• %USERPROFILE%\Pictures (the fallback in get_win_folder_from_env_vars()): https://github.com/adrg/xdg/blob/master/README.md#xdg-user-directories
• For the registry name My Pictures, I used regedit and found this name there.
  • The path: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders

[kemzeb]

The most recent snapshot adds the similar logic of user_documents_dir() and user_pictures_dir() in unix.py into a helper function called _get_user_media_dir(). This will make it easier to introduce other user media directories.

[ofek]

Awesome stuff!