ActivityMonitor: Add swap space to the default memory graph.

I was doing some analysis of the memory areas the kernel was using
after a long uptime, and found that there were 1.3GB worth of
"slab area"s, but the kernel was "only" using 550MB according to
ProcessController ... but then I realized I wasn't looking at
swap, which showed another 700MB of usage.

So, adding it to ActivityMonitor's main memory graph seems to
make sense then. At least most Linux system monitors do this.

src/apps/activitymonitor/ActivityWindow.cpp

@@ -366,6 +366,7 @@ ActivityWindow::_AddDefaultView()
 			// The first view defaults to memory usage
 			view->AddDataSource(new UsedMemoryDataSource());
 			view->AddDataSource(new CachedMemoryDataSource());
+			view->AddDataSource(new SwapSpaceDataSource());
 			break;
 		case 2:
 			// The third view defaults to network in/out