Add support for webkit's run-file-chooser signal

Hey. I added support for webkit's run-file-chooser signal, allowing users to replace the default GTK file chooser with something different (or just keep the file chooser from being shown).
I'm hoping that someone builds file chooser that has previews and supports vim-like shortcuts (a chrome page, maybe?).

Well, back to the code. That were some of the first lines of C I ever wrote, but I'm pretty sure that I didn't fuck it up completely.... ;)

I'll probably built some basic integrated file-manager to test it out tomorrow...

Good night :)