Fix #122895: Frame change does not work in VSE mixed view

Add timeline version of tweak tool to timeline+preview mixed view.

This was removed in 8c53a18c48. Previously tools combined keymaps for
both preview and timeline, but in combined view preview operators were
disabled.

Pull Request: https://projects.blender.org/blender/blender/pulls/122992

scripts/startup/bl_ui/space_toolsystem_toolbar.py

@@ -3765,6 +3765,10 @@ class SEQUENCER_PT_tools_active(ToolSelectPanelHelper, Panel):
             _defs_sequencer_generic.blade,
         ],
         'SEQUENCER_PREVIEW': [
+            (
+                _defs_sequencer_select.select_timeline,
+                _defs_sequencer_select.box_timeline,
+            ),
             *_tools_annotate,
             None,
             _defs_sequencer_generic.blade,