Split travertino declaration module (beeware#3195)

Split the travertino declaration module into a style module and a properties module.

Author: HalfWhitt

changes/3195.removal.rst

@@ -0,0 +1 @@
+Travertino's ``declaration`` module has been split into two smaller modules, ``properties`` and ``style``. Toga's imports have been updated to the new locations, but users with Toga <= 0.4.8 that update Travertino to 0.5.0 (and anyone who may be accessing these classes in user code) will get a ``DeprecationWarning`` explaining the situation.

core/src/toga/style/pack.py

@@ -35,9 +35,10 @@
     TRANSPARENT,
     VISIBLE,
 )
-from travertino.declaration import BaseStyle, directional_property, validated_property
 from travertino.layout import BaseBox
+from travertino.properties import directional_property, validated_property
 from travertino.size import BaseIntrinsicSize
+from travertino.style import BaseStyle
 
 from toga.fonts import (
     FONT_STYLES,

core/src/toga/widgets/base.py

@@ -4,8 +4,8 @@
 from typing import TYPE_CHECKING, Any, TypeVar
 from warnings import warn
 
-from travertino.declaration import BaseStyle
 from travertino.node import Node
+from travertino.style import BaseStyle
 
 from toga.platform import get_platform_factory
 from toga.style import Pack, TogaApplicator

dummy/src/toga_dummy/utils.py

@@ -1,9 +1,9 @@
 from unittest.mock import Mock
 
 import pytest
-from travertino.declaration import BaseStyle
 from travertino.layout import BaseBox
 from travertino.size import BaseIntrinsicSize
+from travertino.style import BaseStyle
 
 
 class EventLog: