Automatically find element controller class by name

[OldStarchy]

This is more of a question than a feature request as I'm sure someone has already thought about this before...

Page controllers are found by self::class + 'Controller', why don't we do this for elements too? I'm definitely going to be following this convention when writing controllers for custom elements as that's what we do for PageController, so I think it makes sense that the default getController would check for this class too rather than just going straight for ElementController.