rename header files from *.h to *.hpp

src/widgets/titlebar.hpp

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <QWidget>
+
+namespace chatterino {
+namespace widgets {
+
+class TitleBar : public QWidget
+{
+public:
+    TitleBar(QWidget *parent = nullptr);
+};
+
+}  // namespace widgets
+}  // namespace chatterino