Update the field widths on demand in wxStatusBarGeneric.

Update the field widths on demand in wxStatusBarGeneric.

This ensures that the value returned from GetFieldRect() is always up to date,
even when this method is called from the user-defined wxEVT_SIZE handler, i.e.
before our own OnSize() could run.

Also remove the now unneeded hack with calling the base class OnSize() from
the statbar sample.

Closes #14268.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71365 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775