Tweak the size of the multiline text edit popup dialog

This caps its size on large displays. This in turn prevents lines from
becoming very long, which could hamper text readability.

editor/editor_properties.cpp

@@ -116,7 +116,7 @@ void EditorPropertyMultilineText::_open_big_text() {
 		add_child(big_text_dialog);
 	}
 
-	big_text_dialog->popup_centered_ratio();
+	big_text_dialog->popup_centered_clamped(Size2(1000, 900) * EDSCALE, 0.8);
 	big_text->set_text(text->get_text());
 	big_text->grab_focus();
 }