AppletContainer binding for containments

This adds a class to manipulate applets in more detail in context of
containments. The type will only be available from Containments, not
Applets or PopupApplets.

This binds the necessary functionality of Applets to build fully
features containments. The code has been moved from MobileComponents,
untangling another piece there and putting the functionality where
it belongs, into the specific component type bindings to allow this
functionality also outside of plasma-mobile and further reduce the
delta between the shell implementations.

REVIEW:106817