Add testsupport QML API and error page signals

The testsupport API is used for test those features that are not
planned to be accessible via the public API. For example,
signals of the chromium's error page should not be visible for the
develepor but it is important to test whether the error page is loaded
properly.

Change-Id: Ie5b3265a207d05649cabd624a001a6ffe2e45971
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>