title: set fix size of an java rcp4 Part
body: <p>I prgram an application with Java and use RCP4.
  Now I´d like to program an navigation Bar in my application.
  The navigation Bar shoud have a fix width which shoud not be modifiable.</p>
  
  <p>Can I do thas with a Part in the RCP4 application model?
  I found no way to fix the size of a Part.</p>