Renamed ImportDialog VoxelImportDialog
Remove UI behavior from VoxelImporter class

This commit removes QT behavior from the VoxelImporter class so that
it may be used as a general utility in the application.

- The voxel tree reference used for importing has been moved from `VoxelImporter` to `Application`. Placing it next to the `_clipboard` tree.
- `ImportDialog` has been renamed `VoxelImportDialog`
- Dialog settings loading has been moved into `VoxelImportDialog` from `VoxelImporter`
- The new method VoxelImporter::validImportFile(const QString&) can check whether a file is valid for voxel import
- `VoxelImporter::import(const QString&)`
Corrected signals, SUPPORTED_EXTENSIONS list, defaulted VoxelImporter to clipboard