Move .linked class from buttons to their container

According to gtk's documentation, GTK_STYLE_CLASS_LINKED has to be set
on the surrounding container instead of on the widgets that should be
linked.

Also move setting the style classes for toolbar buttons into the .ui
file.

https://bugzilla.gnome.org/show_bug.cgi?id=726167
header: allow window dragging for clicks in empty space

When clicking on the area allocated to song information text,
but not over or near the text, start window drags the same way
GtkToolbar does, taking the window-dragging style property into
account.

https://bugzilla.gnome.org/show_bug.cgi?id=678938