main: show tooltip with instructions when no pass has been entered

This adds a new tooltip that shows when the app starts, and when no text is in the input box (including when all text is deleted), informing the user to input a disk decryption passphrase.

The tooltip disappears as soon as the user starts entering characters, and the normal password dots replace it in the input box.

I decided to use a tooltip in this way since there was not enough space on the screen to fit it elsewhere when the screen height is small (e.g. on the N900), and I think this looks rather nice.

fixes #60 (closed)

screenshot