Explicitly add clips and tracks to their respective containers

[sakertooth]

Currently, we add clips and tracks to tracks and track containers in their base classes. As a consequence for adding it to the container before the object has completely finished constructing, the application can crash at selective moments due to the use of an incomplete object (most commonly by usage from the audio thread).

To fix this, callers must now add clips and tracks to their respective containers explicitly after construction has completed.

This is a continuation of #7594, just without being too ambitious to avoid bugs. There is most likely a lot more refactoring that can be done here to make resource management a bit more safe.

[messmerd]

I haven't tested it but it looks good to me

[sakertooth]

I have made some changes, in particular, making addClip, removeClip, addTrack, and removeTrack all virtual. Now, any behavior that depended on the clip or track being added to the parent in the constructor can happen within these functions as extendable behavior on top of the normal functions.

My next goal, which I might not achieve in this PR, is to remove the Track* and TrackContainer* parameters from the constructors to prevent misuse (e.g. passing in a track to the constructor of a Clip, but then adding it to another track by accident). Edit: Requires sweeping changes, will leave for another PR.