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

docs/changes.txt

@@ -445,6 +445,7 @@ All (GUI):
 
 - Added support for showing bitmaps in wxButton.
 - Added wxInfoBar.
+- Added stretchable spaces support to wxToolBar.
 - Added support for corner, row and column headers renderers to wxGrid.
 - wxWindow::SetAutoLayout() now works for all windows, not just panels.
 - Support wxListCtrl columns, items and image lists in XRC (Kinaou Hervé).