Password dialogs

[Jonathan Blandford <jrb redhat com>]

Hey guys,

I added password support to evince this evening.  I'm not super thrilled
with how the code turned out, but it seems to work.  ev-window.c could
use a round of refactoring to clean it up.  As per most of the features,
only pdf is supported as I think this feature it unique to it.

There's one weird case that I'm not handling now.  If you open a
document with a password, and then cancel the dialog, we just sit there
with a 'blank' window, and with no way to get the password dialog back.
Bryan Clark proposed placing a message in the blank page, letting them
know that the document is encrypted, and including a button to bring the
password dialog back up.

I'm happy to try this sometime, but it's going to make the ev-window
code even more messy, as we can now have a document half-loaded.  I
don't know if anyone has a better suggestion or not.  Thoughts?

Also, there's now a TODO file in the code.  Please feel free to fill it
in.

Thanks,
-Jonathan