Enhancement: add the ability to just disable rules

[Giova84]

Sometimes could be useful to disable some "rules", but actually is only possible by deleting them: in a case of a complex rule, we will have to reinsert again every part of a "formula" which composes the rule.
So would be nice to add a "enable/disable" option for every rule.

[owenca]

I guess the simplest way, from the GUI point of view, is to insert a Disable button between Edit and Remove in the Rules tab. The rules will be grayed out when disabled. What do you think?

[humdingerb]

Sounds good. The button label should toggle between "Disable" and "Enable" depending on the selected entry.
Maybe even add a "-- disabled" after the rule name in the list, to make it even more obvious.

With 4 buttons below the list, the window gets quite wide. I suggest switching their location with the "Move up/down" buttons, i.e. the Move buttons below, the Add/Edit/etc. vertically arranged to the right of the list.

[owenca]

Is there a way to delete the above links to the intermediate (and squashed) commits? Please use the PR link instead.

[humdingerb]

Fixed with #18 bd41f3d. Thanks owenca!

[Giova84]

I'd want to say a big thank you to owenca for his good work and improvements on this useful application!