began UI for keyframing

Largely extending the existing NodeParamView with functionality from the
previous release.

app/node/input.cpp

@@ -27,6 +27,7 @@
 
 NodeInput::NodeInput(const QString& id) :
   NodeParam(id),
+  keyframable_(true),
   keyframing_(false),
   dependent_(true),
   has_minimum_(false),
@@ -241,6 +242,16 @@ void NodeInput::set_is_keyframing(bool k)
   keyframing_ = k;
 }
 
+bool NodeInput::is_keyframable() const
+{
+  return keyframable_;
+}
+
+void NodeInput::set_is_keyframable(bool k)
+{
+  keyframable_ = k;
+}
+
 const QVariant &NodeInput::minimum() const
 {
   return minimum_;