Move dropdown button text and icon code to dropdown button class

include/Theme.hpp

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <mglpp/graphics/Color.hpp>
+
+namespace gsr {
+    struct GsrInfo;
+
+    struct Theme {
+        Theme() = default;
+        Theme(const Theme&) = delete;
+        Theme& operator=(const Theme&) = delete;
+
+        mgl::Color tint_color = mgl::Color(118, 185, 0);
+    };
+
+    void init_theme(const gsr::GsrInfo &gsr_info);
+    const Theme& get_theme();
+}