Added m_pageInitialized flag to avoid change-events during
initialization

Change-Id: I40fcbf275779da2249e7ac2e410e551bd71e41b1
Signed-off-by: André Wegmüller <awe@bsiag.com>
Reviewed-on: https://git.eclipse.org/r/38151
Tested-by: Hudson CI
Reviewed-by: Claudio Guglielmo <claudio.guglielmo@bsiag.com>