Simplify fn Events::handle_resize

[dhardy]

Use a simpler API signature. Allow EditBox to assume its contents do not need size_rules or set_rect called again (they don't in this case, though usually calling ConfigCx::resize() indicates otherwise).

Alternative considered: make ActionResize an enum and all use-cases specify the type of resize required. But: it's considerable extra complexity and in almost all other cases the two variants would be equivalent. This might be worth revisiting if further changes to the layout-solving API take place.