Set callback for bookmark gengrid item instead of items thumbnail.

Set callback for bookmark gengrid item instead of items thumbnail.

[Issue#]    https://bugs.tizen.org/jira/browse/TT-135
[Problem]   Clicking on bookmark folder text doesn't work.
[Cause]     Callback was set on thumbnail instead of on whole item.
[Sollution] Move callback to bookmark gengrid item click.
[Verify]    1. Run browser
            2. Create some bookmark folders
            3. Open More Menu -> Bookmark manager
            4. Click on some bookmark folder description
            5. Bookmark folder shold be opened.

Additional changes:
  - Changed some function names to make code more self-explaining.
  - Removed redundant focus managment callbacks causing problems
    in the current approach.

Change-Id: Ibddc457a7cfdc19f7b68ce4163f4b2ae04d793db