Add patch touched property

Open reuk requested to merge reuk/lv2:feature/reuk/patch-touched into master

This is an initial proposal for a method of communicating parameter 'touched' information for parameters which aren't backed by control ports.

An advantage of this approach is that it's very easy to implement for plugins/hosts which already implement patch parameters.

One potential issue is that the patch:touched property seems to overlap a little with the patch:writable property (the parameter should not be writable while it is touched). Perhaps an alternative approach would be to allow 'writable' to be enabled/disabled dynamically by the plugin on specific parameters. I'm not sure how those patch messages would look, though, and this might be confusing for uses such as MIDI learn (it's not intuitive to check for parameters which became read-only in this case...).

I'm also not sure how the release procedure works, which is why I haven't updated the changelist in patch.meta.ttl.