Allow previewing non-drawable scene objects with drawable children

[dgelessus]

The "Preview" context menu item for scene objects was only enabled if the scene object had a draw interface directly attached. This PR implements previewing scene objects where there's no draw interface on the object itself, but some of the children do have draw interfaces.

Here's how this looks for Ahnonay for example:

I don't know the PlasmaShop codebase very well, but I didn't have to do much to get this working. QPlasmaRender already supported rendering multiple objects at once, so it was just a matter of recursively going through the parent object's children and adding every scene object to the renderer.

This doesn't work perfectly yet though. The code that sets the initial camera perspective only looks at the parent scene object and not the children, which isn't much help if the parent isn't drawable. At best, the camera starts zoomed too far in, and at worst, it lands in the middle of nowhere, far from the actual objects. Not sure how to fix this - the vector/matrix math in QPlasmaRender::center goes a bit over my head.

[dgelessus]

Also added a previewer for entire scene nodes in the same way. It has the same issues as my other change, but still works relatively well in many cases. Here's some Er'cana for example:

[zrax]

Please rebase to get the CI fixes from #77

[dgelessus]

There are still some problems here with certain objects not being positioned/scaled correctly. For example, if you preview the spyroom scene node, the telescope and book appear far off from the rest of the geometry. I'm not sure what exactly causes this and how it could be handled better - I don't fully understand all the ways that Plasma stores object positions and geometry...

[dpogue]

I don't fully understand all the ways that Plasma stores object positions and geometry...

For static objects, the matrix is usually baked into the geometry data, but for dynamic objects (anything animated or kickable and including clickables, probably) the matrix is stored in the CoordinateInterface (and then they can get complicated the chains of children inheriting parent matrices).