Update path_provider plugin

[provokateurin]

The path_provider got updated a lot in recent times and got some new functions and even desktop exclusives. We need to update the plugin accordingly.
https://github.com/flutter/plugins/blob/master/packages/path_provider/lib/path_provider.dart
When I have time I'll create a PR in the next days.

[pchampio]

@jld3103 what is your plan for the PR?
I've looked at the new channels method_channel_path_provider.dart some of them could indeed be merged into the plugin.

I think the following package can be useful https://github.com/adrg/xdg

[provokateurin]

I didn't work on that, because the project I needed it for was changed in a way that the plugin was no longer in use, but I could actually update the plugin in the next days.