Draft for async loading

[xarvic]

This is a sketch for async loading of view sequences.

ViewSequence currently acts as the interface between views which do layout and strategies for loading views. This PR tries to add a way for layout views like LinearLayout to request the parts of a ViewSequencewhich would be visible while keeping the the identity for a specific view which should stay in focus.

It adds a set_focus method to the ViewSequence trait. This method can be used by sequences with a variable length to load only the elements which would be visible.
The rebuild method is changed to return a SequencePosition this marks the index of the element marked in set_focus in the rebuild sequence. Using this the container could call a method like druids scroll_area_to_view() to keep the the element in view.

This is a idea i had for some time, which i would like to discuss in the office hours. The implementation for the backend is missing and the design for SequencePosition also needs some work.