Allow vertical tabs and more control about the tabs placement

  * This adds two new settings to org.gnome.Epiphany.ui
    - expand-tabs-bar controls whether the tabs should fill the
      available space in the bar.
    - tabs-bar-position controls the position of the tabs bar
      (GtkNotebook)

  * The default value is to keep the current UI (tabs bar on the top
    and expand the tabs to fill the space).

  * To configure vertical tabs, tabs-bar-position can be set to left
    and expand-tabs-bar to false. Setting expand-tabs-bar to true
    when tabs-bar-position is left or right can be also useful
    for touch devices.

https://bugzilla.gnome.org/show_bug.cgi?id=764368