Modified the code related to the sidebar

- The ScrollbarHideType does not appear to be
  set anywhere in the application, also hiding
  the scrollbar does not seem like a good idea.
- Escape key closes the search bar
- The sidebar size is implemented using the
  position of the GtkPaned instead of a size
  request of a widget
- The buttons for copying / deleting pages
  are now implemented in the glade file