Tabindex

[skjalgepalg]

Differentiate between focusable elements:

• Native focusable elements updated to match spec
• Programatic focus (all native plus any set tabindex)
• Keyboard focus (all native plus nun-negative tabindex values)

Use programatic focus selection for initial focus, use keyboard focus selector when keeping focus inside dialog

Resolves #645

[mittinatten]

LGTM!