Rework extensions setup and dependency

[Kryptos-FR]

When an extension to the pipeline is not selected, the corresponding renderers should not be included. Additional styles should also live separately from the common styles.

So for example TaskList is an extension (see PR #3 ). It declares a new style key TaskListStyleKey that it uses for CheckBox. And it also has its own renderer TaskListRenderer.

When setting up the pipeline and the WpfRenderer we should query which extensions are available and only include the corresponding renderers.