Add the entered method to the UIScreen class

[poncovka]

The UI screen doesn't have a method that would be called every time a user enters the screen. It is possible to use a workaround with the setup method and the screen_ready property set to False when the screen is closed (see the example [0]), however it is not a very friendly solution. The new method would be useful for updating the initial configuration of the screen before it is shown to the user.

Also, I am not sure why the setup method is even supported. It seems to be too specific without any immediate benefit to the simpleline library. It feels like a leftover from anaconda.

[0] https://github.com/rhinstaller/python-simpleline/blob/master/examples/03_entry_counter/03_entry_counter.py#L65