Add a default mode event to ServerSideDecorationManager

Summary:
The ServerSideDecorationManager gains a new event which gets sent
to the client when it binds the manager. The event indicates the
default server decoration mode used by the server. This allows the
client to know the decoration mode when it creates a decoration and
thus does not need to roundtrip to the server to get the mode.

Reviewers: #plasma, sebas

Subscribers: plasma-devel

Projects: #plasma

Differential Revision: https://phabricator.kde.org/D1129