Follow up of #204371 (exploration done in #207699).

This issue is about:

• When available, use EditContext to react on user input
• Use existing text area or new div elements to render content for screen-reader users

Advantages:

• Very likely fixes many IME bugs
• Simplifies IME handling in code

Disadvantages:

• EditContext is not available on all platforms, in particular not on electron by default
• EditContext does not improve accessibility directly (indirectly, by allowing to use divs to render the screen reader content)
  • text area code is complicated and needs to be adjusted for screenreader support