Add support for stretchable spaces to wxToolBar.

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