Added a searchbar style to Controls.TextBox.xaml.
Merge pull request #575 from Amrykid/search-textbox-style

MetroSearchTextBox Style
Added ContentTemplate to DataGridHeaderColumn for easier styling

Now you can define your own contenttemplate for the header so you can also use it without to upper converter if you want to
Created a circular button style for the ToggleButton.
Use AccentColor for the outer ring.