#552 Docking window thumbnail placement

[Donat Callens]

The docking of plugin windows (or core functionality windows, like File Browser or Hypersearch Results) in jEdit could be visually improved by allowing to put the tumbnails in a horizontal position for lateral docking, and vertical posotion for top/bottom docking. This placement would increase screen real estate by reducing the width of lateral docking, and the heigt of top/bottom docking.