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/widgets/window.hpp

@@ -25,7 +25,7 @@ class Window : public BaseWindow
 public:
     enum WindowType { Main, Popup, Attached };
 
-    explicit Window(singletons::ThemeManager &_themeManager, WindowType type);
+    explicit Window(WindowType type);
 
     void repaintVisibleChatWidgets(Channel *channel = nullptr);