Addon Manager: Refactor installation code

Improve testability of installation code by refactoring it to completely
separate the GUI and non-GUI code, and to provide more robust support
for non-GUI access to some type of Addon Manager activity.

src/Mod/AddonManager/Init.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 # FreeCAD init script of the AddonManager module
 # (c) 2001 Juergen Riegel
 # License LGPL