Fix for scrolling the active row visible in diff page

When using keyboard navigation keys in the diff page, the active row
was not automatically scrolled into view in Chrome and IE. Firefox did
not have the same problem, though. The 'scrollIntoView' function from
'com.google.gwt.client.Element' class just doesn't work correctly
in those two browsers. Problem fixed by implementing an own version
for scrolling the window to correct position.

Task-number: QTQAINFRA-903
Change-Id: I170fa0e4d2f45fcab95e2af08ed9530b71c1ee15
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Ismo Haataja <ismo.haataja@digia.com>