vsgQt refactor

[robertosfield]

Hi All,

The last major bit of work i did on vsgQt was to refactor it to support multiple Windows and MDI usage, which work nicely under Linux but once users on Windows started tested they found issues. After lots of investigation and experiments with possible solutions to this issue I have come up with refactor of vsgQt that changes how the frame loop is managed. The PR that pulls in these changes is PR 30.

The changes do unfortunately mean we've had to remove the the initialize callbacks that vsgQt::ViewerWindow original used, so if you still use this old API you'll need to update your code. The vsgqtviewer, vsgqtmdi and vsgqtwindows should help point the way how you set things up.

To get things to work cleanly vsgQt now uses:

1. vsgQt::Viewer subclass from vsg::Viewer to add Qt specific management of when rendering is invoked and events
2. vsgQt::Window subclasses from QWindow that "has a" vsgQt::Viewer (previous you could assign a vsg::Viewer.)
3. vsgQt::Window "has a" vsg::Window to provide the vk/vsg::Instance, vk/vsg::Device,vk/vsg::Surface support.

When doing multiple windows, with with an MDI or as independent windows you create a single vsgQt::Viewer that is shared between each Window.

While the integration with the VSG and Qt is a bit awkward because of the way Qt works I think it's now a bit neater than it was original, as well as being more flexible and hopefully robust. Testing out in real-world applications is what will really tell us how close vsgQt is to being in settled state, so please test :-)

There are still remaining issues though that I can resolve at this point, and will need help with:

1. Integration with macOS is broken right now, I don't have the knowledge/available hardware to resolve this so need input from the community.
2. Support for non XCB windowing under Linux, such as Wayland and X11 which will need additions to the core VSG.
3. QML support has been requested, but I have zero knowledge of QML nor any projects I am working on that require it, so we'll need help from the community to make this happen.

I am happy to push ahead with a vsgQt-0.2.0 release to wrap up the latest changes but will wait for testing feedback from the community before I tag this release.