Add wxColour::{Set,Get}RGB[A]().

These methods allow to operate with all 3 or 4 colour channels at once.

Add their implementation, documentation and a unit test for wxColour
exercising them.

Closes #9918.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61976 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

tests/test_test_gui.dsp

@@ -243,6 +243,10 @@ SOURCE=.\events\clone.cpp
 # End Source File
 # Begin Source File
 
+SOURCE=.\graphics\colour.cpp
+# End Source File
+# Begin Source File
+
 SOURCE=.\controls\comboboxtest.cpp
 # End Source File
 # Begin Source File