Make it possible to have implementations in different toolkits.

Also, make it possible to share the headers.

gtk/OGBox.m

@@ -0,0 +1,42 @@
+#import "OGBox.h"
+
+@implementation OGBox
++ box
+{
+	return [[[self alloc] init] autorelease];
+}
+
+- init
+{
+	self = [super init];
+
+	@try {
+		if (isa == [OGBox class])
+			@throw [OFNotImplementedException
+			    exceptionWithClass: isa
+				      selector: _cmd];
+	} @catch (id e) {
+		[self release];
+		@throw e;
+	}
+
+	return self;
+}
+
+- (void)appendChild: (OGWidget*)child
+	     expand: (BOOL)expand
+	       fill: (BOOL)fill
+	    padding: (float)padding
+{
+	gtk_box_pack_start(GTK_BOX(widget), child->widget, expand, fill,
+	    padding);
+}
+
+- (void)prependChild: (OGWidget*)child
+	      expand: (BOOL)expand
+		fill: (BOOL)fill
+	     padding: (float)padding
+{
+	gtk_box_pack_end(GTK_BOX(widget), child->widget, expand, fill, padding);
+}
+@end