Check for WebKit xwidgets inside all xwidget-webkit functions

Check for WebKit xwidgets inside all xwidget-webkit functions

This is done in preparation for the introduction of other
xwidgets, such as media xwidgets, even though there are
only WebKit widgets at present.

* src/xwidget.c (CHECK_WEBKIT_XWIDGET): New macro.
(WEBKIT_FN_INIT): Use CHECK_WEBKIT_XWIDGET.

(Fxwidget_webkit_search, Fxwidget_webkit_next_result)
(Fxwidget_webkit_previous_result)
(Fxwidget_webkit_finish_search)
(Fxwidget_webkit_load_html): Check that xwidget is a WebKit
widget.