Add ´__repr__´ to ´Control´

[ndonkoHenri]

More on __repr__ here.

Example:

>>> import flet as ft
>>> print(repr(ft.BottomSheet(show_drag_handle=True, visible=False, enable_drag=True)))
BottomSheet(visible=False, open=False, enabledrag=True, showdraghandle=True)

Notes:

• without the explicit use of repr(), the __str__ is instead used:

  >>> import flet as ft
  >>> print(ft.BottomSheet(show_drag_handle=True, visible=False, enable_drag=True))
  bottomsheet {'visible': False, 'open': False, 'enabledrag': True, 'showdraghandle': True}

• showdraghandle, for example, is printed instead of show_drag_handle because the camelCase "showDragHandle" passed in the setter is being lowered in Control._set_attr_internal before being added to Control.__attrs - this makes it quite impossible for me when grabing it, to make it snake_case

• the attributes content or controls, are not printed out - obviously because in contrast to the other bool or string attributes, they are not added to the Control.__attrs ; any thoughts?