BUG: 3 fixes for widgets (MultiCrusor, SpanSelector, Slider)

These are fixes for 3 minor widget bugs:

1. Slider was working very slowly on webagg. 

Changing draw to draw_idle fixed it for me. 

2. MultiCrusor was not showing up on webagg.

3. SpanSelector was responding to Pan Axes widget (but should have been locked).

This is my first PR, so I hope it doesn't break anything. ;)

Please let me know if there's anything else I need to do.