Revert "Allow themes to override only a "base" image like"

git-svn-id: svn+ssh://svn.gna.org/svn/gnustep/libs/gui/trunk@36481 72102866-910b-0410-8b05-ffd578937521
* Source/GSTheme.m:
* Source/NSImage.m: Revert previous commit and write a much cleaner
implementation that fixes the same bug.

I removed the step in theme activation where we call
+[NSImage _setImagePath:name:] on each image in the theme, and instead
modified +[NSImage _pathForImageNamed:] to also search the theme images
directory.

When a GSTheme activates now, it only calls +[NSImage _reloadCachedImages]
which checks all NSImage cached by name and reloads any whose path has
changed.

git-svn-id: svn+ssh://svn.gna.org/svn/gnustep/libs/gui/trunk@36482 72102866-910b-0410-8b05-ffd578937521