use a font glyph to represent passphrase 'dots' on the screen

This simplifies the way passphrases are represented on the screen by replacing the drawn circle/dot with any specified character from the font in osk.conf.

As an added bonus, users can set the option in the config to "" to have osk-sdl hide any passphrase dots on the screen, which is the same behavior cryptsetup has. Onlookers can't easily see how long a passphrase is.

(the default is just a boring dot: ●)