*WIP* Register Click

Hi all, I'm pushing this up while I'm still in a very early stage. I'm getting stuck on passing optional blocks via the ampersand `&block` syntax. 

I keep getting wrong number of argument errors and am now thoroughly lost :frown:. I may just need a tutorial on passing and using option args . . . more comments in line.
This change makes it so that I *don't* get a wrong number of arguments error in:
```ruby
Shoes.app do
  oval 100, 100, 20, 50, click: proc {alert "!!!"}
end
```
but after clicking the oval the program quits saying that `@block` never got defined [here](https://github.com/shoes/shoes4/blob/master/lib/shoes/swt/common/clickable.rb#L45). So I'm still essentially having an issue with methods not handling arguments the way I think they're handling them.