allow-page-request-open-window

add a new setting to control opening new window made by page request
(window.open javascript or target="_blank"). The user request opening of
new window isn't changed.

The implementation split VB_TARGET_NEW in VB_TARGET_NEW_PAGE_REQUEST and
VB_TARGET_NEW_USER_REQUEST.

Any user action that performed VB_TARGET_NEW is converted to
VB_TARGET_NEW_USER_REQUEST, and others to VB_TARGET_NEW_PAGE_REQUEST.

In vb_load_uri, VB_TARGET_NEW_USER_REQUEST is unconditionnally followed,
whereas VB_TARGET_NEW_PAGE_REQUEST open a new window only if
'allow-page-request-open-window' is 'on' (the default). Else the
behaviour is like VB_TARGET_CURRENT.