gnu: icecat: Update to 60.2.0-gnu1 (pre-release).

* gnu/packages/gnuzilla.scm (icecat): Update to 60.2.0-gnu1.
[source]: Download pre-release from alpha.gnu.org.  Remove obsolete patches.
Comment out the code to delete the bundled copies of libevent, cairo,
harfbuzz, and graphite2.
[inputs]: Use the latest ffmpeg.  Comment out libevent, cairo, harfbuzz, and
graphite2.
[native-inputs]: Add rust and cargo.
[arguments]: Remove --enable-gio and --disable-gnomeui.  Add --disable-stylo.
Comment out --with-system-{libevent,harfbuzz,graphite2}, --enable-system-cairo.
Import %cargo-build-system-modules.  Add 'patch-cargo-checksums' phase.
* gnu/packages/patches/icecat-CVE-2018-5157-and-CVE-2018-5158.patch,
gnu/packages/patches/icecat-bug-1413868-pt1.patch: Delete files.
* gnu/local.mk (dist_patch_DATA): Remove them.