Label for overlay

- clean up buildLabels()
- change height widget item text: "Height from surface" --> "Height from DEM"
use list comprehension

- change classmethod to staticmethod
data-defined color support
features to export and clip geometries options
display the dat-gui panel on the front of labels
one axis rotation for each side
load plugin icon from file directly

qgis2threejs.py:
- move import Qgis2threejsDialog into run()
- do not use import *