Allow customizing AUI tab colours in wxAuiTabArt.

Add wxAuiTabArt::SetColour() and SetActiveColour() methods and provide trivial
default implementation of them in wxAuiDefaultTabArt to allow customizing the
tab colours.

Closes #11411.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67428 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/aui/auibook.h

@@ -340,6 +340,20 @@ public:
     */
     virtual void SetSelectedFont(const wxFont& font) = 0;
 
+    /**
+        Sets the colour of the inactive tabs.
+
+        @since 2.9.2
+    */
+    virtual void SetColour(const wxColour& colour) = 0;
+
+    /**
+        Sets the colour of the selected tab.
+
+        @since 2.9.2
+    */
+    virtual void SetActiveColour(const wxColour& colour) = 0;
+
     /**
         Sets sizing information.
     */