implemented basic animation curve editor widgets

Frontend is largely done, it just needs backend and connecting to other
widgets to function correctly.

app/widget/curvewidget/curvewidget.h

@@ -0,0 +1,25 @@
+#ifndef CURVEWIDGET_H
+#define CURVEWIDGET_H
+
+#include <QPushButton>
+#include <QWidget>
+
+#include "curveview.h"
+
+class CurveWidget : public QWidget
+{
+public:
+  CurveWidget(QWidget* parent = nullptr);
+
+private:
+  QPushButton* linear_button_;
+
+  QPushButton* bezier_button_;
+
+  QPushButton* hold_button_;
+
+  CurveView* view_;
+
+};
+
+#endif // CURVEWIDGET_H