Some editions to the floating watermak

Changed the whole storyboards.
Moved the storyboards to Shared.xaml, to share them in both TextBox.xaml
and PasswordBox.xaml.
![mahapps-floating-message2](https://cloud.githubusercontent.com/assets/4404199/3969675/f8418200-27c2-11e4-8756-2579ca9ab388.gif)