Fixes several issues with grid registration of the dtwm panel on dtfile startup and...

Fixes several issues with grid registration of the dtwm panel on dtfile startup and on workspace addition:

  The panel registration code rather stupidly assumed that display
  size is always 1280x1024 pixels. Because of this, depending on screen
  size, the panel could be registered somewhere in the center of the screen
  or completely or partially beyond of it.

  The panels were registered only on startup, not  those, which were added
  from UI.

The fix moves panel registration into separate routine and removes assumption
about display size. The fix yet is not complete since it still makes assumption
about panel's geometry: from dtfile there is still no way to find out
dinamically  the size of panel and it's location.