Added focusable_children property

tests/renderables/test_tint.py

@@ -0,0 +1,17 @@
+import io
+
+from rich.console import Console
+from rich.text import Text
+
+from textual.color import Color
+from textual.renderables.tint import Tint
+
+
+def test_tint():
+    console = Console(file=io.StringIO(), force_terminal=True, color_system="truecolor")
+    renderable = Text.from_markup("[#aabbcc on #112233]foo")
+    console.print(Tint(renderable, Color(0, 100, 0, 0.5)))
+    output = console.file.getvalue()
+    print(repr(output))
+    expected = "\x1b[38;2;85;143;102;48;2;8;67;25mfoo\x1b[0m\n"
+    assert output == expected