MetroDemo overhaul

I think that we all agree when I say that the current demo for Mahapps.Metro is crap.

Because of this, I'm currently working on a complete overhaul of the demo that is more usable.

This pullrequest is still a work in progress, I opened it for feedback :)

## Current status:

### Buttons

![buttons](https://f.cloud.github.com/assets/964691/548859/14730cbe-c2ed-11e2-8a46-961ff6e492a9.png)

### Text

![text](https://f.cloud.github.com/assets/964691/548860/1e7b85ba-c2ed-11e2-8f60-01a141a77515.png)

### Selections

![selection](https://f.cloud.github.com/assets/964691/548863/289ac5a6-c2ed-11e2-8a9a-5129231fdb0f.png)

### Sliders and progress

![slidersprogress](https://f.cloud.github.com/assets/964691/550491/e24a7f26-c312-11e2-89e5-31b774e5bb9d.png)

### TabControl

![tabcontrol](https://f.cloud.github.com/assets/964691/549692/150be8b0-c2fe-11e2-8048-9cb1aa4efdb4.png)

### Others

![others](http://i.imgur.com/VmY4zxC.png)

## TODO:

**Remaining controls**
- [x] Sliders
- [x] Combobox
- [x] Progressbar
- [x] These other things that move
- [x] Context menus
- [x] Flyout
- [x] ProgressIndicator

**Other things**
- [x] Themes showcase
- [x] Transitions
- [x] Validations
- [x] Link to the VS Demo