Dashboard improvements: add buttons to clear all choppers/detectors

[nvaytet]

Could be useful to have buttons to clear all choppers/detectors in one go.
Note that when implementing this, we porbably also want to cleanup any event connections from the widgets.
So when removing a chopper widget, we need to unobserve_all() (or the button._click_handlers.callbacks = [] equivalent for buttons).

Also, by default, an instrument is loaded (Odin).
There should be an option in the dropdown menu to make an empty instrument that would clear everything and start fresh.