title: Clear MenuItem, with clear command
body: <p>hi 
  i'm a "very" beginner in wpf
  i'm trying to make a menu item "Clear", it should clear the text in the focused text box,
  actually i could not find a built in command that does the job like (copy,paste,cut..etc)</p>
  
  <p>is there one built in or do i have to make a custom routed command, and if so
  i've tried but failed, and need ideas</p>
  
  <p>i've made the ClearCommandExecuted logic, but the problem is with "CanExecute"
  i tried to access the Keyboard.FocusedElement there, but failed because the focused element is the menu item it self when it's clicked !!!!</p>
  
  <p>please help
  thanks</p>