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
cc26010927