chore: remove Singleton & replace getIApp with getApp (#5514)

tests/src/Scrollbar.cpp

@@ -1,7 +1,7 @@
 #include "widgets/Scrollbar.hpp"
 
 #include "Application.hpp"
-#include "mocks/EmptyApplication.hpp"
+#include "mocks/BaseApplication.hpp"
 #include "singletons/Fonts.hpp"
 #include "singletons/Settings.hpp"
 #include "singletons/Theme.hpp"
@@ -17,11 +17,11 @@ using namespace chatterino;
 
 namespace {
 
-class MockApplication : mock::EmptyApplication
+class MockApplication : mock::BaseApplication
 {
 public:
     MockApplication()
-        : settings(this->settingsDir.filePath("settings.json"))
+        : theme(this->paths_)
         , fonts(this->settings)
         , windowManager(this->paths_)
     {
@@ -41,7 +41,6 @@ public:
         return &this->windowManager;
     }
 
-    Settings settings;
     Theme theme;
     Fonts fonts;
     WindowManager windowManager;