[presentation] Allow text corrections by users

[stweil]

It's easy to make the text area editable by adding the HTML5 contenteditable attribute (see tutorial). An additional button is needed on the client side to submit (HTTP POST) the modified text to the server.

More work has to be done on the server side to synchronise the text lines with the ALTO XML, to store the modified ALTO and to make it available for end users or for internal model improvements.

[stweil]

Steps for implementation:

1. Add required attribute unconditionally (trivial change).
2. Add submit button for web page with dummy action on the server side. This is an option, so an instance of Kitodo.Presentation can configure whether OCR results should be editable or not. The submit button and the attribute only exist if editing is enabled.
3. Add a working action on the server side. Details still open.

[csidirop]

Fortunately the last point shouldn't be that hard, because the textlayout is preserved, as one can see here: