Implement Web Share API through Purpose

This implements Web Share API Level 1 [1] through Purpose and also adds a generic "Share..." context menu entry.
It can be tested on [2].

[1] https://w3c.github.io/web-share/
[2] https://w3c.github.io/web-share/demos/share.html

Differential Revision: https://phabricator.kde.org/D23151