webadmin: custom properties sheet: tooltip for key

Show tool-tip for selected key, when hovering on keys drop-down.
Since keys are user defined it can overflow the widget width, therefore
a tooltip is needed.

Change-Id: I2118f78e340923e89e0008c2c8b8bb9c9a19466b
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>