Called by tests and layout linter to create a main window.
Simplifies creation of the main window, we can now remove one of the ctors.
Before we supported both the controller creating the view and vice-versa,
while this is convenient it's complex to support, due to order or initializations.
Now, you a create a view, and it creates the controller behind the scenes.
MainWindow is special in this sense, as it's created by the end user, it's
not something internal we want in the ViewFactory.
this removes QWidget references from ClassicIndicators.cpp.
Both the QtQuick and QtWidgets impl will implement this new
interface, which is what ClassicIndicators.cpp uses now
