This change makes it possible to reuse the generic window decorator
classes in include/decorator/ for decorators of a different structure.
E.g., instead of painting decorations on a single nitpicker session,
each window may paint its decorations into additional window-specific
nitpicker sessions.
This patch improves the decorator in two ways. First, it enables the
assignment of window colors depending on the window labels. This
configuration can be changed dynamically. Second, it adds the handling
of window controls for closing, maximizing, minimizing windows.
Issue #1689Fixes#1688
Norman Feske