Fix kickoff accessibility and rewrite key/focus handling

In the old code sometimes the root item would mostly have focus,
sometimes the relevant object. The root item would then perform key
handling for everyone and dispatch events to the relevant place.

Because focus kept ending up in the wrong place and faked, this broke
accessibility (as well as being quite messy).

In the new code items all handle their own key handling. Anything
unhandled propagates to the root view which then Keys.forwardTo the
relevant items.

Behaviour is mostly identical (except I changed escape in the
Applications view)

Change is too massive for 5.11, will delay for master.

Test Plan: Orca tells me tab when I change, and item when I change item

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D7780