Initial implementation of the GObject signal wrapper system.

Initial implementation of the GObject signal wrapper system.
* Implement classes Signal and SignalHandler that wrap the basic functionality of
  the g_signal* methods, including signal connection and emission and using both
  variadic and non-variadic template versions for the connect() and emit() methods.
* Refactor the Closure system and add non-variadic versions of the templates for
  compilers that do not support C++0x variadic templates.
* Make the ValueBase::get / ValueBase::set methods throw exceptions instead of
  asserting. This helps showing nicer error messages from the signal system when
  these methods are called from there and allows in some non-fatal cases to
  continue execution.