Dynamic loading/unloading needs to be supported

Several customers have indicated that they want to have dynamic loading/unloading (of slides) supported in Qt 3D Studio runtime to manage both startup time (only load what is needed) and to keep memory usage in check (only keep things that are required in memory). The basic requirement is to add an API for customers to say "load this slide to memory" and to say "unload this slide from memory". The implementation of the logic what is loaded/unloaded is use case dependant and will be done by the customer using these two simple APIs. In addition we probably need an API for loading a presentation so that only the master slide and the default slide are loaded in to memory. Also signalling from the runtime on when slides are loaded/unloaded might be needed.