Debugger: Remove enclosing quotes when editing strings in watchwindow.

When doubleclicking to edit a string value displayed as '0x3443 "foo"'
in the watchwindow, the whole display value including quotes
was set to the line edit. Remove them.

Change-Id: I0327e3e0fa2270047c8c1b28b07ad5bc5e531644
Reviewed-on: http://codereview.qt.nokia.com/1771


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Christiaan Janssen <christiaan.janssen@nokia.com>
Reviewed-by: Kai Koehne <kai.koehne@nokia.com>