Cleanup: move PlayheadSnappingPanel out of tool-system module

This panel is unrelated to the tool-system.

scripts/startup/bl_ui/space_sequencer.py

@@ -18,8 +18,11 @@ from bl_ui.properties_grease_pencil_common import (
 )
 from bl_ui.space_toolsystem_common import (
     ToolActivePanelHelper,
+)
+from bl_ui.utils import (
     PlayheadSnappingPanel,
 )
+
 from rna_prop_ui import PropertyPanel