core: Rework ModuleView code using Kirigami Platform's Units

Units was moved to the KirigamiPlatform library, so account for that. While we're at it, also fix the usage of units here as in its current state it's completely wrong. Units has been a plugin for a while now, creating just a Units instance is going to create a fallback instance that doesn't actually make use of the right plugin.

Using Units::create() with nullptrs is still somewhat wrong as we really shouldn't be using this type without a QmlEngine, but at least it will try to load the right plugin.