Share button for headlines and support for OS share functionality

[schoettl]

Use case:

As a supervisor in a company, I want to be able to share header items (via e-mail, instant messenging, or other smartphone apps) to request more information on that item or to send information to the assigned employee.

Solution:

• [x] Next to the edit, tags, +, ... buttons of a headline, there should be a share button.
• [ ] Clicking this button should start the OS share system (like in Android, where you can select Telegram, WhatsApp, Gmail, ... to share a photo)
• [ ] The headline with all its text body should be shared. Including subheadlines, which can still be deleted before sending.
• [ ] Also, filename and line number should be shared, in case the other party is supposed to update the item in the org file.

The share system depends on the operating system (Android, iOS, Desktop, other). I don't know yet how to use such share systems.

-> https://w3c.github.io/web-share/ not sure if Smartphone OS already support that.

As a fallback, there is already the mailto: action.

[munen]

@schoettl FYI: There's prior work regarding native share functionality in this issue: https://github.com/200ok-ch/organice/issues/52

In that issue, the idea is to get things shared into organice, though. In any case, there might be overlap in the implementation.

@dotcs Pinging you here so that you know that there's another interested party in implementing native share functionality.

[dotcs]

What I have implemented in the PoC in #52 is a receiver for the Android (and maybe iOS?) system intent when a text/link is shared. I'm not sure if this overlaps with triggering such an event on the mobile OS, but as soon as I find some time (which is likely next year) I'll try to understand the issue better. I like the idea of this PR. So in the future it might be possible to share from and to organice. That's a great idea.

[schoettl]

I change this issue to aim for a future support of the OS share systems.