`InputFocus` fields are no longer public

The .0 field on InputFocus is no longer public. Use the getter and setter methods instead.

In 0.18:

let focused_entity = input_focus.0;
input_focus.0 = Some(entity);
input_focus.0 = None;

In 0.19:

let focused_entity = input_focus.get();
input_focus.set(entity);
input_focus.clear();

Additionally, the core setup of InputFocus and related resources now occurs in InputFocusPlugin, rather than InputDispatchPlugin. This is part of DefaultPlugins, so most users won't need to make any changes.