NSBox: (re-)implement isOpaque (#254)

This (re-)implements logic where a box is marked
as opaque if it is a custom box which is not
transparent, but moves the logic to `GSTheme`,
allowing themes to override this loggic.

This fixes the rendering of custom boxes with a
custom fill color; the current implementation
would mark them as non-opaque and hence the fill
color would not be rendered.

- The original implementation (from 1999) always
  returned `YES`
  (53bcb50)
- A more complete implementation was backported
  from the Testplant branch in 2012
  (e85b16b, by
  @fredkiefer, upstream commit
  36e77b9)
- More recently, the a return value of `NO` was
  hardcoded
  (02bc49e, by
  @ericwa)

Headers/Additions/GNUstepGUI/GSTheme.h

@@ -1339,6 +1339,8 @@ APPKIT_EXPORT_CLASS
 		 clipRect: (NSRect)clipRect
 		   inView: (NSView *)view;
 
+- (BOOL) isBoxOpaque: (NSBox *)box;
+
 - (void) drawBoxInClipRect: (NSRect)clipRect
 		   boxType: (NSBoxType)boxType
 		borderType: (NSBorderType)borderType

Source/GSThemeDrawing.m

@@ -3475,6 +3475,18 @@ - (void) drawTableViewRow: (NSInteger)rowIndex
     }
 }
 
+- (BOOL) isBoxOpaque: (NSBox *)box
+{
+  if ([box boxType] == NSBoxCustom)
+    {
+      return ![box isTransparent];
+    }
+  else
+    {
+      return YES;
+    }
+}
+
 - (void) drawBoxInClipRect: (NSRect)clipRect
 		   boxType: (NSBoxType)boxType
 		borderType: (NSBorderType)borderType

Source/NSBox.m

@@ -464,18 +464,7 @@ - (void) drawRect: (NSRect)rect
 
 - (BOOL) isOpaque
 {
-  // FIXME: Depends on theme; if always returning NO is a performance hit
-  // we can check if GSTheme is going to draw an old-style opaque box
-  // or not.
-  return NO;
-  // if (_box_type == NSBoxCustom)
-  //   {
-  //     return !_transparent;
-  //   }
-  // else
-  //   {
-  //     return YES;
-  //   }
+  return [[GSTheme theme] isBoxOpaque: self];
 }
 
 - (NSColor*) fillColor