LibVT+Terminal: Don't set window title directly from TerminalWidget

Instead, have TerminalWidget provide an on_title_change hook.
This allows embedders to decide for themselves what to do if we receive
a "set terminal title" escape sequence.

Applications/Terminal/main.cpp

@@ -163,6 +163,9 @@ int main(int argc, char** argv)
 
     RefPtr<CConfigFile> config = CConfigFile::get_for_app("Terminal");
     auto terminal = TerminalWidget::construct(ptm_fd, true, config);
+    terminal->on_title_change = [&](auto& title) {
+        window->set_title(title);
+    };
     window->set_main_widget(terminal);
     window->move_to(300, 300);
     terminal->apply_size_increments_to_window(*window);

Libraries/LibVT/TerminalWidget.cpp

@@ -323,10 +323,8 @@ void TerminalWidget::paint_event(GPaintEvent& event)
 
 void TerminalWidget::set_window_title(const StringView& title)
 {
-    auto* w = window();
+    if (on_title_change)
-    if (!w)
+        on_title_change(title);
-        return;
-    w->set_title(title);
 }
 
 void TerminalWidget::invalidate_cursor()

Libraries/LibVT/TerminalWidget.h

@@ -43,6 +43,8 @@ public:
 
     virtual bool accepts_focus() const override { return true; }
 
+    Function<void(const StringView&)> on_title_change;
+
 private:
     // ^GWidget
     virtual void event(CEvent&) override;