Use small notification icons when available

Summary:
The notification plugin now sends small notification icons, if the large
icon is not available.

Before patch, with icon:

Before patch, without icon:

After patch, without icon:

Test Plan: I tested notifications with small icons and those with large icons, and it works as expected.

Reviewers: KDE Connect, VDG, fabianr, albertvaka

Reviewed By: KDE Connect, VDG, fabianr, albertvaka

Subscribers: albertvaka, mmustac, fabianr, apol, nicolasfella

Differential Revision: https://phabricator.kde.org/D10165