[Notifications] Take the screen geometry into account when positioning popup notifications

Summary:
The availableScreenRect is relative to the current screen, so if the notification
applet is not on the top-left screen, the notification popup will not appear next
to where the applet is but on the top-left screen instead. Adding the absolute
screen geometry to the availableScreenRect ensures that the popup is positioned
on the right screen.

Test Plan:
Position two screens above each other, place the panel on the bottom screen, the
notification now correctly appears on the bottom screen next to the applet.

Reviewers: broulik, davidedmundson, lbeltrame

Reviewed By: broulik, davidedmundson, lbeltrame

Subscribers: lbeltrame, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21184