webadmin: introduce dropdown action button for tabs

* Created a dropdown action button for tabs.
* Will be used for "Preview -> Custom..." button in snapshots sub-tab.
* For illustration, see screenshot in [1].

[1] http://www.ovirt.org/Features/Single_Disk_Snapshot

Change-Id: If3aab713e0ac40f5bf95d0df9a7e9eebbc61f678
Signed-off-by: Daniel Erez <derez@redhat.com>