ui: made icon buttons self updating on color scheme change

* fixed hatching not replaced in single file view title bar on color scheme change

src/lib/settings/ColorScheme.cpp

@@ -18,6 +18,11 @@ ColorScheme::~ColorScheme()
 {
 }
 
+bool ColorScheme::hasColor(const std::string& key) const
+{
+	return isValueDefined(key);
+}
+
 std::string ColorScheme::getColor(const std::string& key) const
 {
 	return getValue<std::string>(key, "#FF1493");

src/lib/settings/ColorScheme.h

@@ -19,6 +19,8 @@ public:
 	static std::shared_ptr<ColorScheme> getInstance();
 	virtual ~ColorScheme();
 
+	bool hasColor(const std::string& key) const;
+
 	std::string getColor(const std::string& key) const;
 	std::string getColor(const std::string& key, const std::string& defaultColor) const;