Change exception handling API

Change exception handling API

This patch changes the exception handling API in the engine slightly, encapsulating
any use of direct throw statements and catch blocks with concrete types. In the future
we need to be able to change the way these are implemented, in order to ensure that
the correct stack unwinding code is triggered for throw and re-throw.

This patch separates the C++ exception object thrown from the V4 exception
(that includes value, throwing context pointer) and stores the latter inside
the engine.

In order for that to compile, ExecutionEngine::StackTrace and StackFrame had to
move into the QV4 namespace directly.

In addition the syntax for catching exceptions changes from

try {
    ...
} catch (QV4::Exception &ex) {
    ex.accept(context);
    QV4::ScopedValue exceptionValue(scope, ex.value());
}

to

try {
    ...
} catch (...) {
    QV4::ScopedValue exception(scope, context->catchException());
}

Context::catchException() checks if there's a "current" exception in the engine,
and if not assumes that we caught an unrelated exception and consequently re-throws.

partiallyUnwind() is also gone and replaced with rethrowException(), in order to
encapsulate the re-throw.

Lastly, in the future nesting try/catch blocks isn't going to be possible due to
limitations in the common C++ ABI with regards to foreign exceptions.

Change-Id: Ic81c75b057a2147e3176d8e0b4d326c14278b47d
Reviewed-by: Lars Knoll <lars.knoll@digia.com>