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.cpp

@@ -0,0 +1,16 @@
+#include "widgets/helper/InvisibleSizeGrip.hpp"
+
+namespace chatterino {
+
+InvisibleSizeGrip::InvisibleSizeGrip(QWidget *parent)
+    : QSizeGrip(parent)
+{
+    // required on Windows to prevent this from being ignored when dragging
+    this->setMouseTracking(true);
+}
+
+void InvisibleSizeGrip::paintEvent(QPaintEvent *event)
+{
+}
+
+}  // namespace chatterino