Enable selecting opacity in generic wxColourPicker

This is implmentation of the feature which is already implemented in
the native wxColourPicker under wxGTK.

docs/changes.txt

@@ -330,6 +330,7 @@ All (GUI):
 - Add support for the events from multimedia keys (Jens Göpfert).
 - Allow suppressing warnings from wxImage::LoadFile().
 - Allow customizing wxRibbon highlight colours (wxBen).
+- Enable selecting opacity in generic wxColourPicker.
 
 wxGTK:
 

interface/wx/clrpicker.h

@@ -8,6 +8,7 @@
 #define wxCLRP_USE_TEXTCTRL       (wxPB_USE_TEXTCTRL)
 #define wxCLRP_DEFAULT_STYLE      0
 #define wxCLRP_SHOW_LABEL         0x0008
+#define wxCLRP_SHOW_ALPHA         0x0010
 
 wxEventType wxEVT_COLOURPICKER_CHANGED;
 
@@ -33,6 +34,9 @@ wxEventType wxEVT_COLOURPICKER_CHANGED;
     @style{wxCLRP_SHOW_LABEL}
            Shows the colour in HTML form (AABBCC) as colour button label
            (instead of no label at all).
+    @style{wxCLRP_SHOW_ALPHA}
+           Allows to select opacity in the colour-chooser (effective under
+           wxGTK and wxOSX).
     @endStyleTable
 
     @beginEventEmissionTable{wxColourPickerEvent}

src/generic/clrpickerg.cpp

@@ -64,6 +64,7 @@ bool wxGenericColourButton::Create( wxWindow *parent, wxWindowID id,
     m_colour = col;
     UpdateColour();
     InitColourData();
+    ms_data.SetChooseAlpha(style & wxCLRP_SHOW_ALPHA);
 
     return true;
 }