RTE that loads xinha on-demand

The RTE plug-in should show the html initially in read-only mode, after clicking on it the user can edit the text (and thus the xinha component is activated).
Ideally the read-only mode looks similar like today and users won't notice the difference. If feedback is needed to make it clear that you have to click first, discuss with Rolf.

We want to avoid scrolling / page movements when the RTE goes to editable mode.
Preferably we show the toolbar (disabled) in read-only mode.

After leaving the RTE it goes back to read-only mode to save resources. (tbd)

also see RDMP-200

• fixed height for preview (with or without scrollbar): same algo as Xinha AutoSize
• Show Xinha with fewer repaints (preferably only one)