Signal support for scratchpads

[Nooo37]

We've had that idea before, I'll communicate it in this issue so that we don't forget about it and in case someone is interested in contributing.

It would be cool to have scratchpads support some basic signals such as open and close so that the user might intervene in some scratchpad actions and customize it even further from there on. For example the use case of an "option to disable the client floating state when closing the scratchpad" as mentioned in #63 should be achievable by doing something like my_scratchpad:connect_signal("close", function(c) c.floating = false end) which would be very intuitive to awesomewm users.

Adding basic signals would probably not even be that hard. We could just keep a list of functions for each signal as an object property that are run when the scratchpad opens/closes. connect_signal would then just add a function to the corresponding list. There are some implementation details to work out though but the concept should be clear.

[Nooo37]

Has been added in #82