Support variadic methods on dialogs

[gkoz]

DialogButtons trait allows to take fixed-length arrays of button definitions generically over their length. Add Dialog::with_buttons and add_buttons that take such an array. While there, publish Dialog::new. Make FileChooserDialog::new take button arrays too. Update the examples.

[GuillaumeGomez]

To wrap the variadic arguments, I was just thinking about doing it with a macro (like print! and stuff). But very nice job ! I'll merge it while waiting for a potentially better way.

[gkoz]

Yeah, I'd like a better way too and the implementation isn't pretty at all. Hopefully we can improve it.

[GuillaumeGomez]

Sure we can and we will !