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

interface/wx/toolbar.h

@@ -228,10 +228,27 @@ public:
         platform so it can be a vertical line (MSW, some versions of GTK) or
         just an empty space or something else.
 
-        @see AddTool(), SetToolSeparation()
+        @see AddTool(), SetToolSeparation(), AddStretchableSpace()
     */
     virtual wxToolBarToolBase* AddSeparator();
 
+    /**
+        Adds a stretchable space to the toolbar.
+
+        Any space not taken up by the fixed items (all items except for
+        stretchable spaces) is distributed in equal measure between the
+        stretchable spaces in the toolbar. The most common use for this method
+        is to add a single stretchable space before the items which should be
+        right-aligned in the toolbar, but more exotic possibilities are
+        possible, e.g. a stretchable space may be added in the beginning and
+        the end of the toolbar to centre all toolbar items.
+
+        @see AddTool(), AddSeparator(), InsertStretchableSpace()
+
+        @since 2.9.1
+     */
+    wxToolBarToolBase *AddStretchableSpace();
+
     //@{
     /**
         Adds a tool to the toolbar.
@@ -525,6 +542,17 @@ public:
     */
     virtual wxToolBarToolBase* InsertSeparator(size_t pos);
 
+    /**
+        Inserts a stretchable space at the given position.
+
+        See AddStretchableSpace() for details about stretchable spaces.
+
+        @see InsertTool(), InsertSeparator()
+
+        @since 2.9.1
+     */
+    wxToolBarToolBase *InsertStretchableSpace(size_t pos);
+
     //@{
     /**
         Inserts the tool with the specified attributes into the toolbar at the