Panel stack and siblings

[fb71]

After ticket #1 has been implemented handling of the "siblings" in the panel stack needs rework. Problems:

• activating more than one sibling leads to insane panel stacks
• closing panels sometimes crashes
• code is somewhat clumsy

Would be cool to have:

• plugable panel open/close behaviour
• default: one child per panel; opening another child closes the current sibling
• straightforward code

[fb71]

commit bbc06d2082e670d15347545899b615f041a299a0

• PanelOps removed altogether
• simple straightforward implementation of the panel stack/sibling behaviour in DefaultAppmanager
• custom implementations may override methods in order to extend/change behaviour (as in DefaultAppDesign)
• seems to work so far :)
• DefaultAppDesign: scrolling comented out because of issues of layout/preferred width