Add ShareAction & ShareMenu to make writing Purpose-enabled apps easier

Currently, if an application wants to integrate Purpose, it has to duplicate a lot of code. Usually all an application needs is an Action or a Menu to share some data, so let's provide that as built-in Purpose components.

@carlschwan I need your OK to relicense some of your code as LPGL-2.1-or-later. It was previously LGPL-2.1-only so I presume you don't care, but just to be safe :-)

Edit: Carl gave the OK, see his comment