[BUG] Gracefully handle layer deletions

[IgorTatarnikov]

Currently, if a user deletes a layer in the GUI a reference to that layers will be hanging around in self.tile_layers. This will lead to undefined behaviour.

Need to settle on how to handle layer deletions. If a user deletes a tile from the GUI, should that tile still be stitched and fused into the final image? At the minimum when a user deletes all tile layers the widget should reset it's internal state to gracefully close open file handles and clear the internal ImageMosaic class.