css: Avoid borders collision in sidebar

https://bugzilla.gnome.org/show_bug.cgi?id=739836
sidebar: Add margin to some areas in sidebar

https://bugzilla.gnome.org/show_bug.cgi?id=739836
sidebar: Set relief none to annotation tool item

https://bugzilla.gnome.org/show_bug.cgi?id=739836
sidebar: Use linked style for bookmark tool button group

https://bugzilla.gnome.org/show_bug.cgi?id=739836
sidebar: Use symbolic icons in sidebar

https://bugzilla.gnome.org/show_bug.cgi?id=739836