Build: Install most headers to Root (and libcore.a/libgui.a)

This makes out-of-tree linking possible. And at the same time, add a
CMakeToolchain.txt file that can be used to build arbitrary cmake-using
applications on Serenity by pointing to the CMAKE_TOOLCHAIN_FILE when
running cmake:

    -DCMAKE_TOOLCHAIN_FILE=~/code/serenity/Toolchain/CMakeToolchain.txt

Toolchain/UseIt.sh

@@ -2,4 +2,5 @@
 DIR=$( dirname $( readlink -e "$0" ) )
 export PATH="$DIR/Local/bin:$PATH"
 export TOOLCHAIN="$DIR"
+export SERENITY_ROOT="$DIR/.."
 echo "$PATH"