#23778 Improved QT Styling

This pull request adds a new method to the engine base class: `_initialize_dark_look_and_feel()` which replaces the `_get_standard_qt_stylesheet()` method (which will still be around however deprecated).

This method sets up a visual environment very similar to the one in maya. It is using a slightly different approach than just using a stylesheet - instead, it is a combination of style, palette and some css on top. The hope is that this should provide a broad, solid and consistent style base for all apps and widgets and ensure that they are looking similar to the way they would look in Maya (or Nuke for that matter).

Engine initialization will typically look something like this with the new method:

```
            # start up our QApp now
            qt_application = QtGui.QApplication([])
            qt_application.setWindowIcon(QtGui.QIcon(self.icon_256))
            self._initialize_dark_look_and_feel()
```

Once released in the core, we can upgrade all the different engines that are currently calling out to `_get_standard_qt_stylesheet` to call this instead. Below is a visual comparison between this new style and maya, on a couple of different platforms (click on it to see full size):

![style sheet comparisons](https://cloud.githubusercontent.com/assets/337710/3257575/fcdc6912-f22d-11e3-89c1-c6231a3c6de5.png)



When this gets merged with the zip branch this should go away as it's already handling the old css: https://github.com/shotgunsoftware/tk-core/blob/25483_core_as_zip/python/tank/platform/engine.py#L717