Function list: support "private functions last" for all programming languages

[zenogantner]

Currently, the "alphabetical_private_last" sorting order refers only to the conventional naming used in - among other languages - Perl.

Padre should use an internal function list representation that indicates whether a function/method is private, so that we can also nicely support languages like Java and C#, where "private" is determined by a keyword.

[zenogantner]

Once this is implemented, we need to modify the last test case in t/9A_java_functionlist.t