[RFC] MetroDialogs v3

This is a refined version of #785.

- [x] Renamed ```MetroWindowDialogExtensions``` to ```DialogManager```. Those who use the extension methods correctly will not be affected.
- [x] Implemented the basic mechanics to show a dialog outside of a metro window.
- [x] Implement events that fire when a dialog is shown.
- [x] Implement final animations for the dialog controls.... somehow.
- [x] The ProgressBar in the ```ProgressDialog``` should use the accent color by default.
- [x] Add support for a third (and forth?) button in ```MessageDialog```.


### Screenshots
![externaldialog](https://f.cloud.github.com/assets/251501/1699156/749436d4-5f84-11e3-815c-702af7ff99c2.PNG)
![md2](https://f.cloud.github.com/assets/251501/1720687/7d7c0492-620a-11e3-8c04-c31068c5e5e9.PNG)