[PATCH] Several rows in taskbar

[Matěj Koubík]

This patch is great. I've tested it with my panel-resizing patch (see
bug #152696) and it behaves well even with unusual sizes of panel.
See screenshots:
http://koulovacka.wz.cz/data/kde-panel-taskbar/01.png
http://koulovacka.wz.cz/data/kde-panel-taskbar/02.png
http://koulovacka.wz.cz/data/kde-panel-taskbar/03.png
http://koulovacka.wz.cz/data/kde-panel-taskbar/04.png