overriding of classes used for save/open panels.
* Source/GNUmakefile: Build new category.
* Source/GSThemeOpenSavePanels.m: Implementation of methods
-savePanelClass and -openPanelClass.
* Source/NSOpenPanel.m: Modifications to use class from current
theme.
* Source/NSSavePanel.m: Modifications to use class from current
theme.
git-svn-id: svn+ssh://svn.gna.org/svn/gnustep/libs/gui/trunk@29791 72102866-910b-0410-8b05-ffd578937521