Add ThemeManager

It can listen to system changes and update the current theme, widgets can use it to get the current theme.

common/themes/theme_manager.h

@@ -0,0 +1,37 @@
+#ifndef THEME_MANAGER_H
+#define THEME_MANAGER_H
+
+#include "theme.h"
+#include "theme_id.h"
+
+#include <QtCore>
+
+class ThemeManager : public QObject
+{
+    Q_OBJECT
+public:
+    static ThemeManager &instance();
+
+    ThemeManager(const ThemeManager &) = delete;
+    ThemeManager &operator=(const ThemeManager &) = delete;
+    ThemeManager(ThemeManager &&) = delete;
+    ThemeManager &operator=(ThemeManager &&) = delete;
+
+    void initialize();
+
+    void setTheme(ThemeId themeId);
+
+    const Theme &getCurrentTheme() const { return currentTheme; }
+
+signals:
+    void themeChanged();
+
+private:
+    explicit ThemeManager();
+    ThemeId themeId = ThemeId::Classic;
+    Theme currentTheme;
+
+    void updateCurrentTheme();
+};
+
+#endif // THEME_MANAGER_H