refactor: Make Account & Settings buttons SVGs (#6267)

instead of rendering the icons directly in Chatterino, they're now SVGs

This has the following consequences:
1. The color does _not_ respect your custom theme. For most users, this
   will not be a problem/concern.
2. You can no longer drag a split onto those buttons to create a new tab

src/widgets/buttons/NotebookButton.hpp

@@ -13,7 +13,10 @@ class NotebookButton : public Button
     Q_OBJECT
 
 public:
-    enum Icon { None, Plus, User, Settings };
+    enum Icon {
+        None,
+        Plus,
+    };
 
     explicit NotebookButton(Notebook *parent);