[Qt][WK2] Add the componentComplete method to WebView

[Qt][WK2] Add the componentComplete method to WebView
https://bugs.webkit.org/show_bug.cgi?id=77111

Reviewed by Kenneth Rohde Christiansen.

Move the touch/desktop initialization to componentComplete.
Also return valid default values in the viewport info functions
if the interaction engine is not yet initialized because the
viewport info component could finish initialization earlier
than the WebView, which results in QML warnings during MiniBrowser
startup.

* UIProcess/API/qt/qquickwebview.cpp:
(QQuickWebViewPrivate::setUseTraditionalDesktopBehaviour):
(QQuickWebView::geometryChanged):
(QQuickWebView::componentComplete):
* UIProcess/API/qt/qquickwebview_p.h:
* UIProcess/API/qt/qwebviewportinfo.cpp:
(QWebViewportInfo::currentScale):
(QWebViewportInfo::devicePixelRatio):
(QWebViewportInfo::initialScale):
(QWebViewportInfo::minimumScale):
(QWebViewportInfo::maximumScale):
(QWebViewportInfo::isScalable):
(QWebViewportInfo::layoutSize):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@106022 268f45cc-cd09-0410-ab3c-d52691b4dbfc