Style Plugin

[gentlegiantJGC]

There should be a plugin to manage registering style sheets and enabling them.
This should replace the hard coded system.

This should support setting the application style set with setStyle as well as a style sheet set with setStyleSheet

The style sheet should use qtsass so that colours can be extracted as variables allowing one plugin to define colours and another to define the style.
This will also allow style sheets to stack on top of each other.

A default style will be needed for the splash screen and exception dialog.

We will need to think about loading and unloading of plugins that extend the style to avoid flickering and application hanging with the style changing multiple times.