* src/xwidget.h src/xwidget.c (valid_xwidget_spec_p): Renamed

* src/xwidget.h src/xwidget.c (valid_xwidget_spec_p): Renamed
`valid_xwidget_p' to `valid_xwidget_p'.
(XWIDGETP): Rename `XXWIDGETP' to `XWIDGETP' and `XXWIDGET_VIEW_P' to
`XWIDGET_VIEW_P'. The old `XWIDGETP' is now implemented solely in
`valid_xwidget_spec_p'.
(CHECK_XWIDGET): New macro to check whether the argument is a xwidget.
(CHECK_XWIDGET_VIEW): New macro to check whether the argument is a
xwidget_view.
* src/print.c src/xdisp.c: Take into account the new names.