webadmin: Fix issues with splitter tabs

- Fixed issue where if the system tree was minimized the
  tab to expand would cover the 'new' button of the active
  main tab.
- Fixed issue where the tab of the splitter would show on
  top of a dialog if the dialog was dragged over it.

Change-Id: I151fc3a76d9c6f55e265688fbecf965326774497
Signed-off-by: Alexander Wels <awels@redhat.com>