title: How can I change the FontFamily in a WPF RichTextBox without modifying previous text
body: <p>When you use the FontFamily property of the RichTextBox it changes the FontFamily of the whole content inside the FlowDocument. 
  The same way you can Execute a command like EditingCommands.ToggleBold, where it only changes the word under the caret or just the new text to be written, there should be a way to do the same thing with the FontsFamilies, and Color.</p>