Option to use default system file picker

[rancidfrog]

The current file picker is not useful, as the name nor the image in the case of images can be seen.
I.E.

An option to use default system file picker can be useful, so that file name can be read or thumbnail can be used to select images.

[gsantner]

The problem is that many (as is most android devices i used eved) don't have any. So making this default is not an option.

if somebody starts with this, both file and folder selection is needed.

[duckbrain]

You could probably fire off the standard intent, and make sure that Markor is a receiver of that intent.

https://developer.android.com/guide/topics/providers/document-provider.html

That way, if you don't have a file picker, it will use Markor, otherwise, Android will ask the user.

[gsantner]

@duckbrain Your suggestion requires #126 to be done first, where contributions are very welcome.