Add input_datetime support for 12-hour input

[arigilder]

Currently INPUT_DATETIME shows a box with 24h input. It'd be really handy to have an input that allows for 12h input with an AM/PM dropdown. I'm specifically looking at a time-only datetime.

Also, maybe related: currently clicking the checkbox doesn't close the dialog, you have to hit X to do that - might be something you want to add.