Move v8 out of qtbase

So long as nothing in qtbase itself needs to use v8, there is no compelling reason for having it (including the git submodule) there. It can either be moved to qtdeclarative, or to its own module.

The only requirement for being able to initiate this move is that mkv8snapshot must still be built as a host tool when cross-compiling. (Currently the host tools appear to be limited to the ones living in qtbase, as decided by the configure script.)

Where to move it? There are two options:

1) Move to qtdeclarative. Since that's also where the new QJS API (QJSEngine et al) lives, it seems sensible.

2) Move to its own module. Since the QtV8 library doesn't provide any public API, only the "raw" v8 API (which is private), this makes sense if there are applications/libraries that want to use v8 directly, without dragging in all of QtDeclarative. (QtWebKit?)

On a related note, there's an issue with working with the Qt sources and the v8 git submodule that won't be completely resolved by this move, wherever v8 will live: Doing "git pull" but forgetting to do "git submodule update". If you forget the last part, there's a good chance v8 and/or qtdeclarative won't build. Also, src/3rdparty/v8 will be marked as modified (containing your old SHA1), and if you do "git commit -a" (because you were working on a patch), you would accidentally revert back to your (old) v8 version. The risk of making that error would be greatly reduced if v8 were moved to its own module, since only the maintainers of QtV8 (which isn't even a public library, remember) would submit changes there.