[Devel] URI define convention

[David Robillard]

Hi kids,

Not having URIs defined in C makes code typo-prone.  To avoid this, I am
going to establish the following convention:

 * All URIs defined by an extension are defined in C with a symbol like
LV2_EXTNAME__LOCALNAME

So, for example, if the "foo" extension defined foo:Thingie and
foo:name, foo.h would contain:

#define LV2_FOO_URI "http://lv2plug.in/ns/ext/foo"

#define LV2_FOO__Thingie LV2_FOO_URI "#Thingie"
#define LV2_FOO__name    LV2_FOO_URI "#name"

The double underscore is to avoid a clash with any other symbols the
extension might define as part of its API, so e.g. LV2_FOO__ is an
unrestricted namespace.

-dr