osx: fix client compatibility

Xcode does not warn when using unavailable APIs on older SDKs.
After recompiling against the 10.8 SDK, errors appeared across the
client.

- NSButton.highlight state has been added in OSX 10.10
- window.beginSheet has been added in OSX 10.9
- window.keyWindow has been added in OSX 10.10
- NSViewController.viewDidLoad has been added in OSX 10.10
- NSString.containsString has been added in OSX 10.10

Change-Id: I9c4e271f49711570859d85d43608cf8edf294e01
Tuleap: #343
diff --git a/src/AppDelegate.mm b/src/AppDelegate.mm
index 7c66b6b..8eb6673 100644
--- a/src/AppDelegate.mm
+++ b/src/AppDelegate.mm
@@ -101,7 +101,7 @@
 
                          // Don't show a notification if we are sending the text OR window already has focus OR user disabled notifications
                          if(qvariant_cast<Media::Media::Direction>(qIdx.data((int)Media::TextRecording::Role::Direction)) == Media::Media::Direction::OUT
-                            || self.ringWindowController.window.keyWindow || !shouldNotify)
+                            || self.ringWindowController.window.isMainWindow || !shouldNotify)
                              return;
 
                          NSUserNotification* notification = [[NSUserNotification alloc] init];