Add button to reset the status of the checklist in the "Show control" panel

[ntamas]

The Show control panel essentially acts as a checklist that you should go through before starting a show. However, when repeating the same show multiple times during a single night, it is somewhat hard to reset the checklist: you need to unload the show file first and then load it again. This risks the possibility of accidentally opening another show file during the process. There should be a button somewhere in the "Show control" panel that allows us to reset the state of the red/yellow/green lights in the panel without unloading the show file.

I don't see immediately where this button should be placed, but the code behind it can be developed already without knowing where the button will be on the UI.