[WIP] Added the floating watermark feature

### [WIP]
A new attached dp in the `TextboxHelper` class, `IsFloatingWatermarkEnabled` is used to allow this new feature in a `TextBox`. The floating watermark uses the global accent color as the foreground.
Closes #1318

Usage:
```c#
xmlns:controls="clr-namespace:MahApps.Metro.Controls;assembly=MahApps.Metro"
```

```c#
<TextBox controls:TextboxHelper.IsFloatingWatermarkEnabled="True" controls:TextboxHelper.Watermark="Some Watermark" MinWidth="100" />
```

![mahapps-floating-message](https://cloud.githubusercontent.com/assets/4404199/3944983/f6e3aacc-2624-11e4-9500-9f4dc4df8894.gif)