Add support for mouse hover state to wxAuiTabArt

This allows wxAuiNotebook to support drawing its tabs differently depending on
whether the mouse is hovering above them or not.

See https://github.com/wxWidgets/wxWidgets/pull/105

include/wx/aui/auibook.h

@@ -99,6 +99,7 @@ public:
     wxBitmap bitmap;      // tab's bitmap
     wxRect rect;          // tab's hit rectangle
     bool active;          // true if the page is currently active
+    bool hover;           // true if mouse hovering over tab
 };
 
 class WXDLLIMPEXP_AUI wxAuiTabContainerButton
@@ -234,6 +235,8 @@ protected:
     wxAuiTabContainerButton* m_hoverButton;
     wxAuiTabContainerButton* m_pressedButton;
 
+    void SetHoverTab(wxWindow* wnd);
+
 #ifndef SWIG
     wxDECLARE_CLASS(wxAuiTabCtrl);
     wxDECLARE_EVENT_TABLE();