Enable (de)selecting all apps in notification filter

Summary:
BUG: 393190

Add checkbox to (de)select all

Old proposal:

Test Plan:
Deselect all, verify that no notification is coming from test app.
Select all, ...
Deselect all, select single app, ...

Reviewers: KDE Connect, broulik

Reviewed By: KDE Connect

Subscribers: albertvaka, kdeconnect, apol, mtijink, broulik, KDE Connect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D12281