Sets the title of the `GtkWindow`.
The title of a window will be displayed in its title bar; on the X Window System, the title bar is rendered by the window manager so exactly how the title appears to users may vary according to a user’s exact configuration. The title should help a user distinguish this window from other windows they may have open. A good title might include the application name and current document filename, for example.
Passing null does the same as setting the title to an empty string.
| this |
a `GtkWindow` |
| title |
title of the window |