and how should such a library function? We need one to do rendering with GTK, one for rendering with Qt, one for rendering OpenOffice/LibreOffice. The same then for each of the different shells oh and people might want to change the window decorations. Yes some flexibility known from KDE Plasma which our users don't want to go away.
Sorry a library just does not fit it. Especially not if we want to be able to experiment from one release to the other.
We just should not try to fix what is not broken and keep server side decorations. Please read http://blog.martin-graesslin.com/blog/2010/05/open-letter... why client side decorations are a very bad idea. And no, most of the issues mentioned in that blog post will not go away with Wayland. Some issues might be fixable by adding some glue there and some hacks there, but then we are again fixing what is not broken :-)