Match the wxQT wxColour with public interface

Now the type of the value returned by Red, Green, Blue and Alpha methods are consistent with wxGTK and wxMSW.
This could fix ticket #16713 (symbols exported)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78313 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775