Implement subclass of CultureInfo (UCultureInfo) to replace ULocale

[NightOwl888]

This issue is blocking ICU4N from going to beta.

In Java, the ULocale class was created as a separate entity in ICU4J because:

1. The Java Locale class is sealed/final
2. In Java, the Locale is stored in a static field, not associated with the current thread like CultureInfo is in .NET

To make ICU4N compatible with .NET, a subclass of CultureInfo should be created (named UCultureInfo, for consistency) to replace ULocale.

This subclass should have all of the functionality of ULocale, but subclassing CultureInfo makes the class automatically compatible with the properties that allow it to be set to the current thread.

Compatibility

Gaps

There are several gaps in functionality between Java/.NET that need to be addressed. Including:

1. Conversion nn-NO-NY to/from nn-NO
2. NumberFormat differences (for example, in Java there is min and max number of decimals, but in .NET there is a single property indicating the number of decimals).

There may be others, more analysis is needed to get the complete list. Perhaps a conversation with the ICU team and/or Microsoft is required to work out how to account for these gaps.

Constructor

The constructor of UCultureInfo needs to accept both standard .NET cultures (in which case, we will just wrap the default culture), and it also should accept ICU-style parameters. A decision needs to be made whether to keep the ICU4N underscores (en_US@collation=phonebook), or to make it more like the .NET style (en-US@collation=phonebook). In the JDK, there are no underscores, the values are passed as separate parameters to the constructor of Locale, so there is no definite precedence to follow.

We may need to take the ICU data into consideration when making this decision.

Calendars

.NET actually has more calendars than ICU, so it is unclear exactly how to deal with this just yet. ICU is meant to be an "up to date" version of Unicode that is more recent than .NET, but we need to check whether the calendars in .NET are being kept up with the standard.

Conversion

Unlike Java, we won't need to convert UCultureInfo > CultureInfo as we are a subclass so UCultureInfo is a CultureInfo already.

To convert the other way, my thought is that we should use the same pattern as was done on the System.Type class in .NET.

var name = this.GetType().GetTypeInfo().Name;

So, we could do a similar thing with CultureInfo by creating an extension method:

var ucultureInfo = CultureInfo.CurrentCulture.GetUCultureInfo();

This would allow us to add additional properties to the subclass for use by ICU4N and/or its users.

Miscellaneous

There are probably additional things that need to be considered. More analysis is required.