fully implemented setting keyframes and standard values in the NodeParamView

Widget now supports creating keyframe and standard values, supports dragging
from sliders (not creating an undo command for each drag), and everything is
undoable.

app/common/lerp.h

@@ -32,4 +32,9 @@ T lerp(T a, T b, double t) {
   return (a * (1.0 - t)) + (b * t);
 }
 
+template<typename T>
+T lerp(T a, T b, float t) {
+  return (a * (1.0f - t)) + (b * t);
+}
+
 #endif // LERP_H