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/Notebook.cpp

@@ -1347,7 +1347,7 @@ void SplitNotebook::addCustomButtons()
     auto settingsBtn = this->addCustomButton();
 
     // This is to ensure you can't lock yourself out of the settings
-    if (getArgs().safeMode)
+    if (getApp()->getArgs().safeMode)
     {
         settingsBtn->setVisible(true);
     }