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.

mocks/include/mocks/EmptyApplication.hpp

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "Application.hpp"
+#include "common/Args.hpp"
 
 namespace chatterino::mock {
 
@@ -9,6 +10,11 @@ class EmptyApplication : public IApplication
 public:
     virtual ~EmptyApplication() = default;
 
+    const Args &getArgs() override
+    {
+        return this->args_;
+    }
+
     Theme *getThemes() override
     {
         return nullptr;
@@ -110,6 +116,9 @@ public:
     {
         return nullptr;
     }
+
+private:
+    Args args_;
 };
 
 }  // namespace chatterino::mock