wpe: Reintroduce persistent WebContext
A WebContext leak was introduced in MR
https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2252.
If we wanted one WebContext per WebView we should also unref the
WebKitWebContext when destroying the WebView.
This patch reintroduces the persistent WebContext, initially part of
https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1484.
Fixes #1084
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1933>