refactor: Make Args less of a singleton (#5041)

This means it's no longer a singleton, and its lifetime is bound to our application.
This felt like a good small experiment to see how its changes would look
if we did this.
As a shortcut, `getApp` that is already a mega singleton keeps a
reference to Args, this means places that are a bit more difficult to
inject into call `getApp()->getArgs()` just like other things are
accessed.

src/widgets/dialogs/SettingsDialog.cpp

@@ -411,7 +411,7 @@ void SettingsDialog::showEvent(QShowEvent *e)
 ///// Widget creation helpers
 void SettingsDialog::onOkClicked()
 {
-    if (!getArgs().dontSaveSettings)
+    if (!getApp()->getArgs().dontSaveSettings)
     {
         getApp()->commands->save();
         pajlada::Settings::SettingManager::gSave();