Improve the whole topObjects/bottomObjects situation

Port topObjects/bottomObjects to required properties

It is weird that Loader's context even worked for those components, because both ApplicationPage and some of them have Bound pragma. At least when it wouldn't work for Components which are declared in the same file, so it might be a QML bug. Either way, context lookup is bad.

Uplift topObjects/bottomObjects properties to AbstractResource as virtual methods

This buy us some type safety at the ApplicationPage which previously relied on JavaScript's mis-feature to silently return undefined value for missing properties, and abused Repeater's interpretation of said undefined value as an empty model.