implemented keyframe type changing

Implemented the UI to change a keyframe's type along with the ValueChanged
signals that should result.

app/widget/keyframeview/keyframeviewundo.cpp

@@ -1,6 +1,19 @@
 #include "keyframeviewundo.h"
 
-KeyframeViewUndo::KeyframeViewUndo()
+KeyframeSetTypeCommand::KeyframeSetTypeCommand(NodeKeyframePtr key, NodeKeyframe::Type type, QUndoCommand *parent) :
+  QUndoCommand(parent),
+  key_(key),
+  old_type_(key->type()),
+  new_type_(type)
 {
+}
 
+void KeyframeSetTypeCommand::redo()
+{
+  key_->set_type(new_type_);
+}
+
+void KeyframeSetTypeCommand::undo()
+{
+  key_->set_type(old_type_);
 }