webadmin: Added ShapedButton class

It represents a custom-shaped button whose faces change according to
supplied image files.

Change-Id: I83eafafa7ca480b350d4b1b96aa560fd184acbad
Bug-Url: https://bugzilla.redhat.com/1000285
Signed-off-by: Lior Vernia <lvernia@redhat.com>