Fix Notifications in Plasmoid

Summary:
Fixed Issues mentioned in https://phabricator.kde.org/T6729
This patch also fixes a crash when the filetransfer of the icon fails

Test Plan:
Receive a notification with a previously unknown icon -> icon in Plasmoid is displayed correctly
Receive a notification with an already existing id -> notification in Plasmoid is updated

Reviewers: KDE Connect, apol

Reviewed By: KDE Connect, apol

Subscribers: albertvaka, broulik, apol, KDE Connect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D7209