Change dummy object atspi role

Change dummy object atspi role

At point accessibility interface propagation stops on atspi with redundant
objects role. This disallows selection of webview accessibility elements, as
background which has that role is set to parent of ewk view.

Change background atspi role to filler.

This also reverts following change:
* c4c4caa Dummy button should be invisible to accessibility

Change-Id: I900651d7458e4b92fb1c89ace2a935d2d00eb2dd
Signed-off-by: Marcin Niesluchowski <m.niesluchow@samsung.com>