KWin scripting goes properties

Dropping the wrapper around Client and just exporting the Client's
properties. This breaks all existing scripts as it's now e.g.:
client.caption
instead of
client.caption()

But the first one is the more natural one for JavaScript and also
for everyone writing QML as well.

Setters and signals are mostly still missing in client. Other parts
like ClientGroup must be converted to properties, too.