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