WIP: Make application initialization and start up more clear

- Create SingleInstance class to handling single instance stuff.
 - Instead of calling getInstance() everywhere, initialize each object
   explicitly in order when application starts and getInstance()s do not
   create instances any more and they merely return the created
   instances.
 - Only set initial values in construtor functions __init__(). Move the
   initialization of context-aware (i.e. things that depend on other
   things) to separate functions.
 - Split application creation and initialziation into several steps and
   them should be called explicitly in the correct order.

cura/Scene/ConvexHullNode.py

@@ -24,7 +24,7 @@ class ConvexHullNode(SceneNode):
         self._original_parent = parent
 
         # Color of the drawn convex hull
-        if Application.getInstance().hasGui():
+        if not Application.getInstance().getIsHeadLess():
             self._color = Color(*Application.getInstance().getTheme().getColor("convex_hull").getRgb())
         else:
             self._color = Color(0, 0, 0)