created directory_dialog for windows and linux

[mattVHartley]

I've implemented a directory dialog that works on windows and on linux,

I've used zenity for linux (implementation is practically the same as file_dialog, albeit with an extra arg) Windows uses SHBrowseForFolder and required the including shlobj.h

[arpit15]

@mattVHartley I extended your implementation to apple. I am wondering what is the use-case of saved_path argument. How about changing that use-case to bool save. This option saves the last traversed path as done in file_dialog?

[wjakob]

Sorry for taking a very, very (!) long time to respond. This looks useful, but to actually consider merging it, there would need to be a macOS code-path as well.