Fix get-tabpane

Check if panel is an instance of JTabbedPane, not
whether its class is JTabbedPane

Signed-off-by: Daniel Borchmann <daniel.borchmann@mailbox.tu-dresden.de>
Rewrote conexp-main-frame in seesaw

Signed-off-by: Daniel Borchmann <daniel.borchmann@mailbox.tu-dresden.de>