core: Bookmarks do not work when Search expr...

Bookmarks do not work when Search expression is not empty

This patch changes the search string to empty string when switching
between the Tree , Bookmarks & Tags in the web admin west pane.

Change-Id: I7bb9e815e84a2ca1c8a740f7e236d16835ddeeef
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1052151
Signed-off-by: Eli Mesika <emesika@redhat.com>