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/common/Args.hpp

@@ -30,6 +30,7 @@ namespace chatterino {
 class Args
 {
 public:
+    Args() = default;
     Args(const QApplication &app);
 
     bool printVersion{};
@@ -60,7 +61,4 @@ private:
     QStringList currentArguments_;
 };
 
-void initArgs(const QApplication &app);
-const Args &getArgs();
-
 }  // namespace chatterino