Avoid black flashes when showing a new QWebEngineView

QPlatformBackingStore still doesn't enable blending when compositing
a QOpenGLWidget with the rest of the widget tree. This means that a
tranparent pixel will show as black.

Until we get to properly go through the work of making it possible to
render a transparent webview, set the glClear color to white in order
to avoid seeing any black background before we get our first frame
from Chromium.

Change-Id: Idf7bb831a3b0d62b02f2527e2a15d830bf1e8ad4
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
Fix "findNext" after having navigated to a new page

The findNext flag tells the frame to search starting from the current
selection, but the code assumes that the flag won't be used when there
isn't any find selection (like when the Frame changed).

Make sure that we won't use that flag after we received a load
committed signal.

Change-Id: I10f0f84226232d150f19fd644d7afe78bc425f74
Reviewed-by: Michael Bruning <michael.bruning@digia.com>