designspaceLib: add add{Axis,Source,Instance,Rule}Descriptor methods

as suggested by Just here:
https://github.com/fonttools/fonttools/pull/1860#issuecomment-601762751

Lib/fontTools/designspaceLib/__init__.py

@@ -1188,15 +1188,35 @@ class DesignSpaceDocument(LogMixin, AsDictMixin):
     def addSource(self, sourceDescriptor):
         self.sources.append(sourceDescriptor)
 
+    def addSourceDescriptor(self, **kwargs):
+        source = self.writerClass.sourceDescriptorClass(**kwargs)
+        self.addSource(source)
+        return source
+
     def addInstance(self, instanceDescriptor):
         self.instances.append(instanceDescriptor)
 
+    def addInstanceDescriptor(self, **kwargs):
+        instance = self.writerClass.instanceDescriptorClass(**kwargs)
+        self.addInstance(instance)
+        return instance
+
     def addAxis(self, axisDescriptor):
         self.axes.append(axisDescriptor)
 
+    def addAxisDescriptor(self, **kwargs):
+        axis = self.writerClass.axisDescriptorClass(**kwargs)
+        self.addAxis(axis)
+        return axis
+
     def addRule(self, ruleDescriptor):
         self.rules.append(ruleDescriptor)
 
+    def addRuleDescriptor(self, **kwargs):
+        rule = self.writerClass.ruleDescriptorClass(**kwargs)
+        self.addRule(rule)
+        return rule
+
     def newDefaultLocation(self):
         """Return default location in design space."""
         # Without OrderedDict, output XML would be non-deterministic.