Add locale support to date and time types

[thiagodp]

Proposal:

1. New property locale

The property locale would assume the document's language by default.

Usage example:

UI Element: Birth Date
  - data type is date
  - locale is "pt"

Every supported language should define the default formats for:

• date
• time
• datetime

2. New function dtf

NOTE: I'm still not satisfied about using a function. Maybe special characters to denote a different kind of string could fit better.

The function means "date time format".

Currently there is the property format, which defines a UI Element's format using a regular expression. The proposed function aims to facilitate that definition by adopting a simpler format, like the PHP date and time format.

Examples:

UI Element: Birth Date
  - data type is date
  - format is dtf( "d/m/Y" )

UI Element: Meeting Hour
  - data type is time
  - format is dtf( "h:i" )

UI Element: Last Update
  - data type is datetime
  - format is dtf( "d/m/Y h:i:s" )

With the proposal, format should assume the locale's format by default. Example:

UI Element: Birth Date
  - data type is date
# format is dtf( `m/d/Y` ) since the language is English and so is the locale.

[thiagodp]

See also #40

[thiagodp]

This feature is being released with Concordia 2. The function dtf was replaced by a new UI Element property called locale format that uses Unicode TR 35 as the date pattern. Example:

UI Element: Start Date
  - locale format is "MM/dd"

The generated value will have only month and day.

[thiagodp]

Locale property released in 2.0-alpha. Function dtf() is not needed anymore, since language expressions were added.