Fix how NewPopupDisposition is exposed in QQuickWebEngineView

I initially misintepreted the meaning of the enum, assuming that it
meant that the window should possibly be blocked. The user_gesture
parameter in WebContentsDelegate::AddNewContents is actually doing
this, while the popup disposition means that JavaScript requested
the window to be opened without one of the standard decoration (i.e.
status bar, menu bar, tool bar, etc.).

Update the QtQuick API to reflect this, renaming the "isPopup"
parameter to a more familiar "isUserInitiated".

The popup disposition is named "dialog" to match the previous
QWebPage::WebModalDialog enum.

Change-Id: Ib0c4bc53671fcf0dd9499aa1be2bbc8c494ba49e
Reviewed-by: Michael Bruning <michael.bruning@digia.com>