Adds unit property to dial.

Adds the ability to store and retrieve a unit string associated with the dial, allowing for more context-aware representation of values.

UI/ui_dial.cpp

@@ -53,6 +53,11 @@ double UI_dial::max() const
     return m_max;
 }
 
+std::string UI_dial::unit() const
+{
+    return m_unit;
+}
+
 void UI_dial::from_json(const json &j)
 {
     UI_item::from_json(j);

UI/ui_dial.hpp

@@ -20,6 +20,7 @@ class UI_dial : public UI_item
 
     double min() const;
     double max() const;
+    std::string unit() const;
 
     void from_json(const json &j) override;
     json to_json(size_t id) const override;