Date control to have placeholder hinting to expected format

[avernet]

For instance, as in:

+1 from user

[avernet]

• Not XBL – This is not implemented in XBL (<xbl:binding element="xf|input:xxf-type('xs:date')">), and I imagine that we wouldn't necessarily need to do the switch for this.
• Possible implementation – Mirroring the current oxf.xforms.format.input.date, we could have a new oxf.xforms.placeholder.input.date property, allowing one to specify the placeholder to use, if any. Similarly, we would also support oxf.xforms.placeholder.input.time.

[avernet]

Since we only support a few possible variations of date and time formats, we wouldn't even need to have property for the placeholder, but could instead infer it based on the current format.

[avernet]

One wrinkle here is that the placeholder needs to be localized. For instance, the placeholder in the above screenshot, "TT.MM.JJJJ", would need to show if the language is German. The thing is that we don't have a system-wide way to store localized resources at the XForms-level. We only have this at the Form Runner-level, and, arguably, this isn't unreasonable.

This brings us back to the idea of using XBL for the date component, as we already do for an xf:input bound to a xs:dateTime (see datetime.xbl).