UI: Rename 'obs' dir to 'UI'

This is to prevent confusion both when prefixing commits and when
reading the directory structure for the first time.

UI/double-slider.hpp

@@ -0,0 +1,22 @@
+#pragma once
+
+#include <QSlider>
+
+class DoubleSlider : public QSlider {
+	Q_OBJECT
+
+	double minVal, maxVal, minStep;
+
+public:
+	DoubleSlider(QWidget *parent = nullptr);
+
+	void setDoubleConstraints(double newMin, double newMax,
+			double newStep, double val);
+
+signals:
+	void doubleValChanged(double val);
+
+public slots:
+	void intValChanged(int val);
+	void setDoubleVal(double val);
+};