Mac: Allow selecting bundles in the disk prefs

Users should be able to select sparsebundles in the disk prefs, but should
still be able to navigate inside .sheepvm bundles.

See cebix#25 (comment)

SheepShaver/src/MacOSX/Launcher/VMSettingsController.mm

@@ -211,7 +211,7 @@ - (void) editSettingsForNewVM: (NSString *) vmdir sender: (id) sender
 - (IBAction) addDisk: (id) sender
 {
   NSOpenPanel *open = [NSOpenPanel openPanel];
-  [open setCanChooseDirectories:NO];
+  [open setCanChooseDirectories:YES];
   [open setAllowsMultipleSelection:NO];
   [open setTreatsFilePackagesAsDirectories:YES];
   [open beginSheetForDirectory: [[NSFileManager defaultManager] currentDirectoryPath]