title: Set font properties in RichTextBox
body: <p>HI, 
  I am developing an editor using <code>RichTextBox</code> in WPF, i have to implement feature that user can set font of selected <code>Text</code> if some text is selected, if nothing is selected then font should be set for new text.
  If i set the font properties(like <code>FontStyle</code>,<code>FontSize</code>) of RTB in later case it will set the properties for whole text, How can i set font properties for new text(i.e if user enter the text it will come with new font setting).</p>