Support multiple CSS files (#1079)

* Support multiple CSS paths

* Update a type to match docstring

* Ensure the demo app still works

* Use absolute paths in tests to (hopefully) appease Windows

* Notes about CSS changes in guide/docstrings, small grammar/typos fixes

* Move snapshot apps into snapshot_tests dir, improve messaging in snapshot output, add test for multiple css files interacting with classvar CSS

* Ensure consistent snapshot naming cross-platform

* Use rpartition instead of partition in import_app

* Fix handling of import_app when colon in arg

* Support paths containing Windows drive names in import_app

* Add note on new relative paths in snap_compare

* Update docs/guide/CSS.md

Co-authored-by: Will McGugan <willmcgugan@gmail.com>

* Fix formatting

* Update CHANGELOG to mention CSS_PATH supporting a list

Co-authored-by: Will McGugan <willmcgugan@gmail.com>

tests/snapshot_tests/snapshot_apps/fr_units.py

@@ -0,0 +1,55 @@
+from textual.app import App, ComposeResult
+from textual.containers import Horizontal, Vertical
+from textual.widgets import Static
+
+
+class StaticText(Static):
+    pass
+
+
+class FRApp(App):
+
+    CSS = """
+    StaticText {
+        height: 1fr;
+        background: $boost;
+        border: heavy white;
+    }
+    #foo {
+        width: 10;
+    }
+    #bar {
+        width: 1fr;
+    }
+    #baz {
+        width: 8;
+    }
+    #header {
+        height: 1fr
+    }
+
+    Horizontal {
+        height: 2fr;
+    }
+
+    #footer {
+        height: 4;
+    }
+    
+    """
+
+    def compose(self) -> ComposeResult:
+        yield Vertical(
+            StaticText("HEADER", id="header"),
+            Horizontal(
+                StaticText("foo", id="foo"),
+                StaticText("bar", id="bar"),
+                StaticText("baz", id="baz"),
+            ),
+            StaticText("FOOTER", id="footer"),
+        )
+
+
+if __name__ == "__main__":
+    app = FRApp()
+    app.run()