Keymap: Add additional scrubbing affordance (Alt-LMB)

This adds an additional way to scrub the playhead, by holding Alt and dragging with the left mouse button, in addition to Shift-RMB.

It's easier to do this, especially with pen input. The other method is still kept, in case you have Emulate 3 Button Mouse enabled, in which case Alt-LMB pans the view

And of course, you can still scrub simply by dragging the playhead, without holding any modifier keys.

Right click select is unaffected.

release/scripts/presets/keyconfig/keymap_data/blender_default.py

@@ -2783,6 +2783,7 @@ def km_animation(params):
     if params.select_mouse == 'LEFTMOUSE' and not params.legacy:
         items.extend([
             ("anim.change_frame", {"type": 'RIGHTMOUSE', "value": 'PRESS', "shift": True}, None),
+            ("anim.change_frame", {"type": 'LEFTMOUSE', "value": 'PRESS', "alt": True}, None),
         ])
     else:
         items.extend([