Automatically set a default window size if unset

This resolves problems when the window manager does not set a default
size. I noticed this while using KDE3's KWin. Previously it went unfixed
because my main window manger always assigns a size. This should resolve
the problem.

  * Added DEFAULT_WINDOW_WIDTH and DEFAULT_WINDOW_HEIGHT
    * DEFAULT_WINDOW_WIDTH = 600
    * DEFAULT_WINDOW_HEIGHT =  800
  * If either height or width is missing from the svte configuration
    in the users directory, use the defaults.