Split up getting the data from the arguments list and actually applying
the values, and move the details of theme creation and application to
better places.

This gets rid of ugly control flow details like that
CorePlugin::initialize created the action manager before calling
parseArguments, because that is needed to apply the presentation mode
argument setting, and parseArguments created the main window because
that needs to be created _after_ setting the theme (which can be
overridden by command line argument), but _before_ applying the override
color argument setting.

Change-Id: I9c99305b6efbfcc4b37cea9e5c70d816a621963b
Reviewed-by: David Schulz <david.schulz@qt.io>