dbus: Fix reading and writing some values and make it more robust

Values are passed to variadic functions as pointers to the value, so the size of the type is very important. There was one mistake passing a float when the callee expects a double. The rest was mostly OK, as dbus_bool_t maps to a 32 bits unsigned integer and we used X's Bool reduces to a plain int which is almost guaranteed to be 32 bits on the platforms where Compiz runs, and signedness doesn't matter for boolean values. The same applies for dbus_int32_t for which we also passed a plain int.

However, consistently using DBus' own types reduces the risk of problems, as well as being formally more correct. I did not change all uses of char* but the ones using a temporary variable where a DBusBasicValue container was already available though, because they map to the exact same type in DBus.