does the framework support keyboard shortcuts for selecting next/previous dockable?
I already use the DockableSelector invoked with Ctrl+Shift+E which is very useful,
but I would also like to give the user the possibility to directly go to the next dockable.
Not at the moment. But there is a global KeyListener available (see CControl.addKeyboardListener), so implementing this functionality yourself should require about 5 lines of code. You could use AbstractCDockable.toFront to transfer focus.
Similar functionality is available for Core, see DockController.getKeyboardController…
Could it be that ctrl+tab is naturally used by the OS or by Swing to change the focus of components? Because the framework would have no chance of beating a key combination that is already in use.