Add a constructor to Text and let the color default to black

We really shouldn't require specifying a color for every Text {} element just in order
to see *some* text.

For Rectangle OTOH transparent is a good default (and thus for Color), hence
this change just to Text.

Right now the constructor bit is also a bit repetitive, this could
perhaps be folded into BuiltinItem to generate the ffi, default impl and
forward to an init function if it exists.

api/sixtyfps-cpp/include/sixtyfps.h

@@ -359,6 +359,11 @@ Flickable::~Flickable()
     sixtyfps_flickable_data_free(&data);
 }
 
+Text::Text()
+{
+    sixtyfps_text_init(this);
+}
+
 namespace private_api {
 template<int Major, int Minor, int Patch>
 struct VersionCheckHelper