Allow users to choose where to save a scene every time.

[Peregrine05]

If a scene has not been saved before, and Save is used, or any time Save as or Save a copy is used, a file explorer dialog box should be opened to allow the user to choose where to save the scene.

This approach may benefit from a consolidation of the separate scene files into one file, as well as the creation of a separate snapshots directory within .chunky that contains subdirectories for each scene, each containing the snapshots.

[leMaik]

Currently the only benefit of having all scenes in the one selected scenes directory is that we can show all scenes.

Splitting that and showing the usual file dialogs as other applications do and only suggesting the scenes directory as a default directory, sounds great.