Added 'HasAlpha' attribute for wxColourProperty. Setting it to true allows user to edit the alpha component.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66520 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/propgrid/property.h

@@ -173,6 +173,12 @@
 */
 #define wxPG_COLOUR_ALLOW_CUSTOM            wxS("AllowCustom")
 
+/**
+    wxColourProperty and its kind: Set to True in order to support editing
+    alpha colour component.
+*/
+#define wxPG_COLOUR_HAS_ALPHA               wxS("HasAlpha")
+
 /** @}
 */
 
@@ -527,6 +533,9 @@ wxPG_PROP_BEING_DELETED             = 0x00200000
     There are various sub-classing opportunities with this class. See
     below in wxSystemColourProperty section for details.
 
+    Setting "HasAlpha" attribute to @true for this property allows user to
+    edit the alpha colour component.
+
     @subsection wxFontProperty
 
     Represents wxFont. Various sub-properties are used to edit individual