[Slayer] Q3DSSurfaceViewer::presentationLoaded not useful

presentationLoaded is emitted at the state where the viewer app and command queue are not yet ready, nor the presentation actually parsed. Thus, despite the naming, the presentation is not actually ready to accept, f.ex., any setAttribute or setDataInputValue calls. This was fixed for early visibility updates in RT2, but a better solution would be to have a signal that actually indicates that the viewer and presentation are ready for action.

Propose renaming this signal to something more descriptive, and add a new signal that indicates when either the viewer app, or command queue is ready to accept commands.

Alternatively, rename Q3DSPresentation::dataInputsReady signal to indicate the latter.(DataInputsReady is already currently emitted at the point when the presentation and associated datainputs are parsed, and are ready to be queried.