Support native browser focus handling

[nikku]

Native browser focus handling makes sure that basic keyboard interaction works (users may tab through the site) and that a basic accessibility level is guaranteed (browser knows which element got focus).

Describe the solution you'd like

We should consider making our toolkit focus aware:

• Make interactive elements focusable
• Selecting elements integrates nicely with native browser focus handling
• Editor actions involving keyboard shortcuts may only be triggered if an actual editor element has focus (cf. forum topic for context).

Describe alternatives you've considered

Leaving stuff as is would be the only reasonable alternative

Additional context

Related to forum topic.

[heiko-folkerts-msg-david]

Being a blind developoer myself and currently facing bpmn in a project I'd like to support this. So you now have at least one user that really needs it though I will primarly read diagrams.