Support the use of an image in UIButton from python code without a theme entry

[aelydam]

Is your feature request related to a problem? Please describe.
No.

Describe the solution you'd like
Let us to make a button to use an image from code instead of setting it up in the theme json.
In the game I'm working, I'm using buttons to create a hotbar where the player can select spells. Each spell has its own icon, and there are some 50+ possible icons.
Setting each possible icon in the theme file and a ObjectID for each spell would be really annoying.
Using code to retrieve each image from the spell database would be much more useful.

Describe alternatives you've considered
As a workaround, I'm setting button.normal_image = my_surface, button.hovered_image = .... and running button.rebuild() and replacing _set_any_images_from_theme in a subclass.
This seems to work, but I'm not sure it would have undesired consequences.

I think implementing this, would open the way for a future implementation of icons for UISelectionList, since behind the scenes each item of a selection list is an UIButton.

[rbaltrusch]

For me, the approach of adding a custom UIImage on top of an empty UIButton widget has worked without needing to specify anything in a theme file, at the expense of needing some additional manual layout work:

manager: UIManager
button = UIButton(
    relative_rect=pygame.Rect(...),
    text="",
    manager=manager,
)
UIImage(
    relative_rect=pygame.Rect(...),
    image_surface=pygame.image.load("image.png"),
    parent_element=button,
    manager=manager,
)