Improve focus/blur and text selection

[krichprollsch]

We currently have a active_element in Document's state for the active input.
But it would be great to improve it to support also blur.

Moreover we need an additional state for the text selection.

• dispatch events on text selection, focus and blur
• listen these events to maintain a document's (or window) state of active element and selected text (I know active_element is in the document's state)
• implement Selection API
• take care of the current selection when filling a text input

Follows #1152