Add share action button to received file notification

Summary: You can share the just-received file. It doesn't work with a custom download directory (just like opening such files doesn't work yet). It also won't close the notification once shared.

Test Plan: Sharing to other apps works.

Reviewers: KDE Connect, nicolasfella

Reviewed By: KDE Connect, nicolasfella

Subscribers: nicolasfella, KDE Connect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D11680