Editor: password input mode

[nicoburns]

A password input should have an underlying string as usual, but should visually render a • (we could also make the character configurable).

[xorgy]

Seems like this maybe can be solved with a font definition... but maybe that's ugly too. I feel like there should be a good place for this.
I don't think it's a good idea necessarily to use PlainEditor for password inputs, since they have very particular design considerations, but maybe something specialized?

[waywardmonkeys]

Possibly complicating things is that some password entries echo the character that you typed for a second or so and then it switches to the obscured character.