Merge branch 'broulik/libnotificationmanager'

This includes:
libnotificationmanager https://phabricator.kde.org/D20265
New notification plasmoid: https://phabricator.kde.org/D20266
as well as dataengine compat: https://phabricator.kde.org/D20490 and https://phabricator.kde.org/D20491

BUG: 222470
BUG: 402144
BUG: 405570
BUG: 391646
BUG: 401819
BUG: 400811
BUG: 392669
BUG: 390143
BUG: 390864
BUG: 374099
BUG: 360990
BUG: 346458
BUG: 398926
BUG: 390152
BUG: 342355
CCBUG: 402391
CCBUG: 399697
CCBUG: 400871
CCBUG: 398580
FIXED-IN: 5.16.0