Added the ability to hide the flyout's nav button

You can now hide the flyout's nav button if you want.
Usage:
Set the new dp "IsNavButtonVisible" of the Flyout class to false. The
default is true.
Added the ability to hide the flyout's title

Usage:
Set the new dp "IsTitleVisible" of the Flyout class to false. The
default if true.

Remark:
Hiding both the title and the nav button in a Flyout will effectively
collapse the flyout's header.
Nav button -> Close button
Added an AppBar sample
Move the dependency properties on the same line
Change the boolean "IsVisible" to properties of type "Visibility" so we're more flexible
Merge pull request #1618 from MahApps/extract-pr-1507

Extracted the Flyout header hiding functionality from PR #1507