[M108 Migration][API] Re-implement ewk_view_visibility_set

[M108 Migration][API] Re-implement ewk_view_visibility_set

1. In w3c spec as [1]"The Page Visibility" specification defines a means
for site developers to programmatically determine the current
visibility of a document and be notified of visibility changes.

Also, it does not mean directly showing or hiding the current webview.

Previous codes have an unnecessary behavior. This patch will just set
visibility to |WasShown| or |WasHidden| without evas_object_show
(or hide) in webview.

And ewk_view_page_visibility_state_set() was already changed by [2]
like current patch.

2. This patch implements ewk_view_page_visibility_state_set.
It just supports 'visible' and 'hidden' for now.

3. In aura, calling WebContents::{WasShown|WasHidden} eventually invokes
RWHVA's ShowWithVisibility() / Hide() functions which has additional
functionality related to page visibility, compared to tizen branch.
Hence make the changes suitable to new architecture.
Also show / hide content image evas object in sync with RWHVA's visibility.

References:
[1] https://www.w3.org/TR/page-visibility/
[2] https://review.tizen.org/gerrit/c/77637
https://review.tizen.org/gerrit/c/277832
https://review.tizen.org/gerrit/c/279282
https://review.tizen.org/gerrit/c/282642

Change-Id: I0e9df5734b3a2e14a3ad82c75e8eaf50b33e368f
Signed-off-by: Bakka Uday Kiran <b.kiran@samsung.com>