Code Review for Job #19616

**Workitem available at https://worklist.net/19616** 

This changeset adds overrides to the `Clipboard` JS scripting interface's `importVoxels()` function:

```js
Clipboard.importVoxels(filename, x, y, z, s);
Clipboard.importVoxels(filename, destinationVoxel);
```

In order to import voxels without opening a dialog I have segregated UI and import behavior (UI logic moved to the renamed `VoxelImportDialog` class and import logic moved to `VoxelImporter`.)