[share] add native file sharing feature

[Kiruel]

Add native file sharing functionality and fix issue from #970

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[ca333]

I signed it!

[ca333]

@ianloic @sethladd can you plz help us - we signed both the CLA but the commit email is diff. We cant change that. Can you set cla:yes plz

[bbedward]

Would love to see this get merged, I've been using it for months now as a git dependency.

[Kiruel]

I signed it.

[Kiruel]

@ianloic @sethladd Hi guys, how many time we need to wait for this PR ? (opening in mars 2019.)

[Kiruel]

Btw I use this change in my main project with no problem during all this months.

[pboos]

Thanks for your fixes :). Didn't see this pull request. Could probably have saved myself time rewriting everything and start off from your branch!
I have updated my pull request #970 and actually fixed a small bug that seems to exist in this code (on iOS related to text + file if I remember correctly).

Please try my branch to see if that one works for you as well. Or if you have any comments there.