extend PBD::ID API to allow construction and operator== using uint64_t

This is theoretically dangerous, because a PBD::ID is supposed to be unique, and this new constructor
cannot guarantee that. However, the same danger already exists with the std::string-based constructor

libs/pbd/id.cc

@@ -34,8 +34,9 @@ uint64_t ID::_counter = 0;
 void
 ID::init ()
 {
-	if (!counter_lock)
+	if (!counter_lock) {
 		counter_lock = new Glib::Threads::Mutex;
+	}
 }
 
 ID::ID ()
@@ -50,14 +51,21 @@ ID::ID (const ID& other)
 
 ID::ID (string str)
 {
+	/* danger, will robinson: could result in non-unique ID */
 	string_assign (str);
 }
 
+ID::ID (uint64_t n)
+{
+	/* danger, will robinson: could result in non-unique ID */
+	_id = n;
+}
+
 void
 ID::reset ()
 {
 	Glib::Threads::Mutex::Lock lm (*counter_lock);
-	_id = _counter++;
+	_id = ++_counter;
 }
 
 bool

libs/pbd/pbd/id.h

@@ -35,6 +35,7 @@ class LIBPBD_API ID {
 	ID ();
 	ID (std::string);
 	ID (const ID&);
+	ID (uint64_t);
 
 	void reset ();
 
@@ -47,6 +48,9 @@ class LIBPBD_API ID {
 	}
 
 	bool operator== (const std::string&) const;
+	bool operator== (uint64_t n) const {
+		return _id == n;
+	}
 
 	ID& operator= (std::string);
 	ID& operator= (const ID&);