Add (invisible) resize handle to frameless usercards and reply threads (#4795)

* feat: add resize handle to usercards&reply threads

* Add changelog entry

---------

Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>

src/widgets/helper/InvisibleSizeGrip.hpp

@@ -0,0 +1,16 @@
+#pragma once
+
+#include <QSizeGrip>
+
+namespace chatterino {
+
+class InvisibleSizeGrip : public QSizeGrip
+{
+public:
+    explicit InvisibleSizeGrip(QWidget *parent = nullptr);
+
+protected:
+    void paintEvent(QPaintEvent *event) override;
+};
+
+}  // namespace chatterino