Fix double clicks needing three presses in QWebEngineView

Since Qt 5.3 QWidget has the same behavior as Qt 4 where it will
replace a second MouseButtonPress event with a MouseButtonDblClick
instead of sending both.

Fix the issue by moving the MouseButtonDblClick ignore code up to the
QtQuick code, assume that upper layers will not send
MouseButtonDblClick events, and re-replace the MouseButtonDblClick
event with a MouseButtonPress in the QtWidgets code.

Change-Id: I529dad2de538f486b00eb900ea6d2ed849a3b1f0
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Fix a crash when attempting to drag

Q_UNREACHABLE in RenderWidgetHostViewQt::GetNativeView is now called
through RenderViewHostImpl::OnStartDragging since the last Chromium
update.

Even after removing this, OnStartDragging will now also try fetching
the device pixel ratio through gfx::Screen::GetNativeScreen().

Provide a dummy implementation of gfx::Screen only to avoid the crash,
the class still doesn't do anything and is filled with Q_UNREACHABLEs
to catch if any of those methods are reached in a future Chromium
update.

Change-Id: I2e9dafab5e4622df97100dd7a859523067635118
Reviewed-by: Andras Becsi <andras.becsi@digia.com>