* src/xwidget.c: Replace xwidget_views by Vxwidget_view_list.

* src/xwidget.c: Replace xwidget_views by Vxwidget_view_list.

xwidget_views was an hardcoded array of fixed lenght of `struct
xwidget_view', Vxwidget_view_list is a list of Lisp_Object.

* src/xwidget.h (xwidget_view): Remove member `initialized'.

This field was only used to determine whether a given xwidget_view was
initialized or not in `xwidget_views'. Now only initialized xwidget_view
are stored in Vxwidget_view_list.