Add support for stretchable spaces to wxToolBar.

Stretchable spaces consume all extra toolbar space not allocated to the fixed
size items. They can in particular be used to right-align (some) toolbar tools.

Add and document the new API, change the sample to show it and implement it
for MSW, GTK and OS X/Cocoa.

Also refactor MSW background erasing/repainting code to avoid duplicated calls
to DrawThemeBackground(), call it from a new helper MSWEraseRect() function.

Note that we may want to add support for "invisible" separators, IOW
non-stretchable spaces. This could be easily done for MSW after the changes in
this commit and is supported natively by GTK+ and Cocoa so implementing this
would be trivial if there is any interest.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62850 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/toolbar.h

@@ -142,6 +142,10 @@ protected:
     // the total number of toolbar elements
     size_t m_nButtons;
 
+    // the sum of the sizes of the fixed items (i.e. excluding stretchable
+    // spaces) in the toolbar direction
+    int m_totalFixedSize;
+
     // the tool the cursor is in
     wxToolBarToolBase *m_pInTool;
 
@@ -149,6 +153,17 @@ private:
     // makes sure tool bitmap size is sufficient for all tools
     void AdjustToolBitmapSize();
 
+    // update the sizes of stretchable spacers to consume all extra space we
+    // have
+    void UpdateStretchableSpacersSize();
+
+    // redraw the background of the given part of the window (or entire window
+    // if the parameter is NULL) to erase separator drawn in it
+    //
+    // return true if the background was erased using DrawThemeBackground()
+    bool MSWEraseRect(wxDC& dc, const wxRect *rectItem = NULL);
+
+
     DECLARE_EVENT_TABLE()
     DECLARE_DYNAMIC_CLASS(wxToolBar)
     wxDECLARE_NO_COPY_CLASS(wxToolBar);