Color and Accent Cleanups

In Colours.xaml and the Accent.xamls there were many unused Brushed, or simply duplicate items that this PR should fix.

* Added AccentBrushes for all Accents and replaced where possible (needed) in controls
* Renamed BraindingBrush to ProgressBrush and changed gradient to HighlightColor -> AccentColor3
* Renamed HighlightDarkColor to HighlightColor (because they where always the same and only HighlightDarkColor was used)
* Commented unused Brushes out (HighlightLightBrush, NavigationBorderBrush, NavigationForegroundBrush, PageBorderBrush, BodyTextColorBrush, HoverHyperlinkForegroundBrush, HoverHyperlinkBackgroundBrush, ItemSelectedBrush, SliderTrackBrush)

About the ProgressBrush, this was never really used in any Accent.xaml except for Blue.xaml they all used the same yellow color. I think the new Gradient just looks better and will work for every color:

(Left new, Right old)
![2013-10-07 14_44_39-mahapps metro - demo application](https://f.cloud.github.com/assets/3532342/1280298/f4039aec-2f4e-11e3-9ab5-27369c94b466.png)![2013-10-07 14_48_06-mahapps metro - demo application](https://f.cloud.github.com/assets/3532342/1280299/f4167784-2f4e-11e3-969f-e3925e39b34b.png)

![2013-10-07 14_45_20-mahapps metro - demo application](https://f.cloud.github.com/assets/3532342/1280301/0251a77e-2f4f-11e3-83b5-ca4249222e70.png)![2013-10-07 14_48_22-mahapps metro - demo application](https://f.cloud.github.com/assets/3532342/1280302/02647e8a-2f4f-11e3-8911-5d897cc4fcc4.png)

![2013-10-07 14_44_22-mahapps metro - demo application](https://f.cloud.github.com/assets/3532342/1280304/0977bd04-2f4f-11e3-8f22-8cbe76fa08cf.png)![2013-10-07 14_47_54-mahapps metro - demo application](https://f.cloud.github.com/assets/3532342/1280305/098af7f2-2f4f-11e3-8cee-0538845a1705.png)

![2013-10-07 14_49_12-mahapps metro - demo application](https://f.cloud.github.com/assets/3532342/1280307/0f14c91e-2f4f-11e3-8ac3-ec5dc44b52ed.png)![2013-10-07 14_48_34-mahapps metro - demo application](https://f.cloud.github.com/assets/3532342/1280306/0f10ae74-2f4f-11e3-9262-50bf0989108b.png)



Optional TODOs:
- [ ] Remove the Unused AppBarButton and the related Brushes (ButtonNormalBackground, ButtonNormalBorder)
- [ ] Move AccentSelectedColorBrush to Colours.xaml
- [ ] Rename Colours.xaml to Colors.xaml (but this will propably break things for other people) or add a Colors.xaml that references Colours.xaml and delete it later