Fix missing button for adding imagery from Asset UI

[r-veenstra]

Closes #701

The Use as Terrain Tileset Base Layer button was missing from the assets UI when selecting an imagery asset to add

Two issues were present

Firstly, the self._asset_type needed to be checking for IMAGERY when checking if the button should be created in asset_details_widget.py

Secondly, is_tileset was being passed a prim instead of a prim path, which caused the function to fail

The button is now visible, and successfully adds to either the first tileset found in the selection, the first tileset in the stage if no selection is present, or creates a new Cesium World Terrain tileset if no tilesets are found.

[lilleyse]

Thanks @r-veenstra