Add possibility to set a custom FileType description on Windows

[chrias]

Hi, i noticed that the file extension description on windows (i don't know what's the official name), it's not customizable, it would be cool if it could be set with a method.

I saw in file_picker_windows.dart that the method fileTypeToFileFilter has static "hints" (audios, all files, files etc.).

Here is the property i want to change:

[miguelpruivo]

I think this could be a nice addition @philenius.

[philenius]

@miguelpruivo I agree with you. I'm confident that this feature is possible on Windows and Linux (not macOS though).

But as of right now, there are two further features / bugs waiting to be implemented: #915 (already working on it) and #954.
Btw, could you please merge #953?

[miguelpruivo]

It's all merged now. Do you need something else @philenius?

[WinXaito]

I'm really interested on this feature.

I want to do that: (here is a example with notepad.exe):

And that look it's not possible with this library.

(Note: for open and save)