bt2: values: remove public `value` getter

bt2: values: remove public `value` getter

The public `value` getter (e.g., `my_int_value.value`) is redundant here
since the `bt2.values` object already act like their Python equivalent.
If you need an `int` value out of a `bt2.IntegerValue` object, for
example, you can just use `int(bt_value)` instead of `bt_value.value`.

We keep the `value` setter because this is used to set the underlying
raw value.

Internally, we keep the private `_value` getter to access the object's
raw value.

In the tests, we copy the value to modify instead of using the `value`
property with this pattern:

    orig_value = copy.copy(value)
    # modify value
    self.assertEqual(value, orig_value)

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>