Add 3d import preview support for multi-import

3D asset import preview is currently only done when importing a single asset. The preview should be also available when importing multiple assets at once.

• Show list of imported assets on the left side of import dialog.
• Clicking on asset on the list changes the preview to that asset.
• Clicking also changes the options to match the options used for importing that asset.
• If any asset has changed options, "Accept" button will say "Import" instead, and pressing it will recreate previews.
• Pressing "Accept" will copy imported components under the project