Avoid string-based connect in QQuickImageBase.

String-based connect is relatively slow, and should be avoided in core
items. This improves performance of the

tst_librarymetrics_performance::instantiation_cached(043) image - empty

test case (approximately halving the time).

Task-number: QTBUG-43096
Change-Id: I02485c515435eceacc95c55f877fc8566e7406d7
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>