notifications: Force wayland QPA plugin

Make sure this always run on Wayland.

Issue: #138
notifications: Notify service manager befor failure

If the notifications daemon detects it's not running with the wayland
platform it fails with an error message that is now notified to
systemd.

Issue: #138
notifications: Enable alpha buffer for windows

Notification windows require alpha buffer.

Issue: #138
notifications: Remove workaround to show windows

We needed to resize the window in order to get a frame event before
the compositor actually showed the window but with latest changes
to qtwayland this seems not necessary anymore.

Issue: #138
notifications: Show all the notifications

We currently don't support a notification queue so it's better to show
all notification windows, but in the future we might want to save
the notifications received and show one window at a time.

For now, in order not to loose any notification, we show them all.

Issue: #138
server: Simplify notifications code

Call mapSurfaces() directly in the configure lambda and schedule the
repaint in mapSurfaces().

Issue: #138
Fix HAVE_SYSTEMD

Use #cmakedefine01 so that we either have 0 or 1, otherwise when
libsystemd-daemon is not found we have HAVE_SYSTEMD undefined.

Issue: #135