QtOptionsPage: Store selected toolchain for debugging helpers

Keep selected toolchain when switching between Qt versions.

Change-Id: I801e507d11b36b01888f016c35196fb098473bbf
Reviewed-on: http://codereview.qt.nokia.com/2183


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>