compositor: Don't connect sceneGraphInitialized()

QWaylandCompositor doesn't have it.
compositor: Assume application window role by default
compositor: Remove obsolete WindowWrapper.qml
compositor: Initial support for workspaces

Add workspaces initial support.
compositor: Set lock, overlay, desktop and background position

Set position according to output geometry.
client: Register session manager as a singleton for QML

Make it accessible from elements easily.
client: Power manager as singleton for QML

Register power manager as a singleton so that QML can access it.
client: Register ProcessLauncher as a QML singleton

Make it accessible from QML elements.
client: Remove ServiceFactory

We don't use it since we have registered services as QML singletons and
have data providers now.