Receive multiple files using 1 notification

Summary:
When multiple files are shared from desktop to android the transfer of the
files should be handled using 1 notification

Depends on: D17081

Test Plan:
Share multiple biggish files from desktop to android
The sharing progress is displayed using 1 notification

Reviewers: KDE Connect, albertvaka

Reviewed By: KDE Connect, albertvaka

Subscribers: kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D17157