Removed HeaderFontSize-DP and used FontSize-DP in TabItem

It is a Little bit confusing for users to use the **HeaderFontSize** Property on a **MetroTabItem**. I have removed that DeplendencyProperty and overwritten the **FontSize** Property of TabItem in order to have the same behaviour like it was. Now a user can set the FontSize of a MetroTabItem directly by using the FontSize property and not the HeaderFontSize anymore.

Hope that will make the process easier.

I know that this would be a breaking Change if anyone use the HeaderFontSize property but I think for new users it would be confusing if they set the FontSize of a TabItem and nothing happens.