When the QMenu is closed or the Purpose job finishes, we make sure to send a reply out and reset the pending reply serial.
However, when we determined on the host side that we cannot share the contents,
we send a reply but don't reset the pending reply serial, having every subsequent purpose request
fail until reloading the extension or restarting the browser.

Differential Revision: https://phabricator.kde.org/D25803

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