new feature: multiple dems in model (only in simple mode), fix for enhancement #4

	modified:   html_templates/template.html
	modified:   html_templates/template_no_sides.html
	modified:   qgis2threejs.py
	modified:   qgis2threejsdialog.py
	modified:   qgis2threejsdialog.ui
	modified:   qgis2threejsmain.py
	modified:   ui_qgis2threejsdialog.py