Remove tab engine code

https://bugzilla.gnome.org/show_bug.cgi?id=707922
Update the style of GtkNotebook tabs

This changes the style to be less visually prominent, a bit flatter,
and hopefully more elegant.

https://bugzilla.gnome.org/show_bug.cgi?id=707922
Don't draw tab gap

Tell GTK+ that we don't require a gap at the bottom of the active
tab.

https://bugzilla.gnome.org/show_bug.cgi?id=707922
Don't inset toolbars

Now that we're using inset tab bars the inset toolbars don't fit
in very well.

https://bugzilla.gnome.org/show_bug.cgi?id=707922
Remove custom arrow from engine

It is now the default in GTK.

https://bugzilla.gnome.org/show_bug.cgi?id=707922