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

src/gtk/toolbar.cpp

@@ -519,6 +519,15 @@ bool wxToolBar::DoInsertTool(size_t pos, wxToolBarToolBase *toolBase)
 
         case wxTOOL_STYLE_SEPARATOR:
             tool->m_item = gtk_separator_tool_item_new();
+            if ( tool->IsStretchable() )
+            {
+                gtk_separator_tool_item_set_draw
+                (
+                    GTK_SEPARATOR_TOOL_ITEM(tool->m_item),
+                    FALSE
+                );
+                gtk_tool_item_set_expand(tool->m_item, TRUE);
+            }
             gtk_toolbar_insert(m_toolbar, tool->m_item, int(pos));
             break;