Inject scripts in parts created by WebEnginePage::createWindow

Previously, when a new WebEnginePart was created from
WebEnginePage::createWindow, the associated page wouldn't work with
KWallet because the required javascript scripts wouldn't have been
injected in it. This happened because the scripts were injected from the
part constructor, but the page was later changed (from
NewWindowPage::acceptNavigationRequest) and the original page was
discarded. This meant that only the discarded page would end up with the
scripts, while the real page wouldn't have them.

To avoid this, the code to inject scripts has been moved to a separate
function which is called both by the part's constructor and from
NewWindowPage::acceptNavigationRequest.