Perform initial refactoring work

Things that were once singletons are no longer singletons, but are
instead stored in the "Application" singleton

Some singletons still remain, and some renaming/renamespacing is left

src/singletons/thememanager.hpp

@@ -14,9 +14,10 @@ class WindowManager;
 class ThemeManager
 {
     ThemeManager();
+    friend class Application;
 
 public:
-    static ThemeManager &getInstance();
+    ~ThemeManager() = delete;
 
     inline bool isLightTheme() const
     {