icu4c api proposal: clone() for public c++ classes where missing

I propose to add clone() functions to (most) public ICU C++ classes that are currently missing them.
This completes the C++ class boilerplate (-> coding guidelines) and helps with operations like 
service registration.

Jitterbugs: 698, 699

For class hierarchies, the clone() function will return a pointer to the base class, which is 
usually only one step removed from UObject. See also 
http://oss.software.ibm.com/pipermail/icu/2003-April/005559.html

Expiration: Thursday, 2003-nov-06

Sincerely,
markus