shell: Set the headerbar as our titlebar and add a close button

- set "All Settings" as title of the headerbar
  (in the overview page)
- set the name of the each panel as title of the headerbar
- remove "hide-titlebar-when-maximizes" property
- add separator and close button to the headerbar
  (set "show-close-button" property as true)

https://bugzilla.gnome.org/show_bug.cgi?id=704217
shell: Use a search bar for the search entry

https://bugzilla.gnome.org/show_bug.cgi?id=704217