title: MVVM WPF - Viewmodel commandbinding
body: <p>Iam using MVVM model  and I have  three usercontrols in a WPF Main window and each usercontrol has datacontext set to different viewmodels in the xaml. 
  The main window is also attached to a different viewmodel in the datacontext. Mainwindow has three buttons and should be bound via command binding,  How can we bind the main window buttons to corresponding usercontrol view model Icommand via xaml?</p>