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.

plugins/CuraEngineBackend/CuraEngineBackend.py

@@ -275,7 +275,7 @@ class CuraEngineBackend(QObject, Backend):
         self.processingProgress.emit(0)
         Logger.log("d", "Attempting to kill the engine process")
 
-        if Application.getInstance().getCommandLineOption("external-backend", False):
+        if Application.getInstance().getUseExternalBackend():
             return
 
         if self._process is not None: