Fix tracking of the caret for some editable text fields that rely on text written to the screen such as the main text area in Balabolka.

When fetching the caret rectangle for tracking in displayModel text, make an rpc call and use GetGUIThreadInfo in-process as Windows' logical to physical coordinate conversion is rather stuffed for cCaret.
Fixes #3901.
Merge commit 'fetch_head' into jp2014.1