Enhancing flyouts and buttons

You'll be able to hide the Flyout's nav button (the arrow) or its title, or both.
Usage:
Hide the Flyout's nav by setting the dp `IsNavButtonVisible` to false. The default is true.
Hide the Flyout's title by setting the dp `IsTitleVisible` to false. The default is true.
Hiding both effectively collapses the Flyout's header.
![capture3](https://cloud.githubusercontent.com/assets/4404199/3945482/52b0cb96-2643-11e4-89c3-23ac5139a5c2.jpg)

#### [WIP] Enhancing buttons with `MetroCircleButtonStyle`.
You can set the buttons text that will be shown below it with the attached dp `UIExtensions.LabelText`.
The color will be the accent color, and this text will be clickable, the whole thing will be clickable.
![capture4](https://cloud.githubusercontent.com/assets/4404199/3945486/b75e2f0c-2643-11e4-8764-8eab76b7f852.PNG)
Open for suggestions.

Resolves #1352

Rename this to `IsCloseButtonVisible`, so it's consistent with the naming of the other properties related to this button