Match the Checkbox and Radiobutton brushes with the Textbox brushes

This PR moved the FocusBorderBrush and MouseOverBorderBrush attached properties back to the ControlsHelper class and enables them to also be set for the Checkbox and RadioButton class

Fixes #1527
Removed the accidentally introduced whitespaces
Merge pull request #1619 from MahApps/fix-checkbox-brush

Match the Checkbox and Radiobutton brushes with the Textbox brushes