Add drag and drop asset import

A standard method of adding files to design software is via drag and drop onto the screen area. It would be great if we could support this for adding assets such as images, fonts and the importable assets (metadata, 3d models).

In the case of assets it would be good if the assets were automatically added to the asset browser and project tree and then these panels are activated so the user can see where the newly added asset is.

in the case of importables (metadata, 3d models) it would be good if the process launched the correct importer.