Add support for xdg-decoration-unstable-v1 in toytoolkit

This would ensure generally useful programs like weston-terminal or weston-eventdemo behave better on compositors which support xdg-decoration. (For example, right now Sway often draws a second titlebar over the titlebar toytoolkit renders; also, the minimize and maximize buttons are meaningless on it. Under KWin, having a native titlebar would provide for a context menu to move the window to another virtual desktop.)

This is related to but does not depend on #472.