Hi,

This patch adds xwidgets support to Emcas. So Emacs can now display GTK
widgets. In particular, it can display webpages using webkitgtk.

Also, I use webkitgtk-2.4 instead of webkitgtk, because xwidgets
requires libwebkitgtk-3.0 instead of libwebkitgtk-4.0 to
build.

Moroever, the size of the closure of Emacs increases from 880.2 MiB -->
1232.4 MiB after adding the inputs.