Enable (de)selecting all apps in notification filter

Summary:
BUG: 393190

Add checkbox to (de)select all

{F5812151}

Old proposal:

{F5810640}

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