Bug 79458

As pointed our by Yael, qquickwebview.cpp uses QFileDialog, which requires the use of QT += widgets when compiling the WebKit2 static library. We should delegate all Widget dependencies into the WebKit1 static library and use them from the WK2 one (that means building WK1 before WK2). The intermediate goal is to compile WebCore and WK2 without Widgets, regardless of the fact that the resulting shared library has a dependency to QtWidgets. By avoiding the dependency in those two static libraries we reduce the risk of accidentally introducing new dependencies.