First attepmts to load extensions.

- Added Foswiki::fetchGlobal() function to fetch global variables from a
module without use of "no strict 'refs'" or 'eval'.

- Added Extensions related exceptions.

Handlers are keywrods describing basic extension functionality. So far the
following are added: extClass, extAfter, extBefore, plugBefore. All are
part of Foswiki::Class 'extension' option.

Another Foswiki::Class option added is 'extensible' to be used by classes
allowing their functionality to be overriden. This option exports
'pluggable' handler to declare overridable methods.

- TBM (to be merged into Item13897) – Foswiki::Class imports 5.14 feature
set into a module. No need to always `use v5.14;' or a kind of.

- TBM Foswiki::Exception::transmute doesn't enforce an exception into
destination class if the exception's class is a subclass of destination.

- Added Foswiki::Extension – base class for extensions.

- Foswiki::Extensions performs registration of some of extension parameters
upon loading stage.

- Fixed erroneous use of fetchVar name instead of fetchGlobal. Thanks to
George Clark for pointing me out.

- TBM? Finding name space glob code has been extracted from
Foswiki::fetchGlobal() into Foswiki::getNS() function. May even replace use
of Devel::Symbols in some cases.

Implemented topoligical sorting to order extensions with correspondance of
their dependencies. In case of a detected circular dependecy all extensions
involved are getting disabled. For those modules being loaded successully
overriding classes are prepared.

…to Item14152

* commit '5fcac85155551731f8a04fdd8c667cb93d558497':
  Item13897: OopsException backward compatibility and bug fixes.
  Item13897: Fixed Foswiki::Class incompatibility with perl 5.20 and lower.
  Item13897: Fixed feature :5.14 not been applied sometimes.
  Item13897: Merge of Foswiki::Exception.pm from Item14152
  Item13897: Fixed a problem with @argv being emptied.
  Item14150:  Recompute validation keys on cached pages

…4152

…to Item14152

* commit '883e7cb2ae207df6b817710d1f9f33c20b4c8247':
  Item13897: Fixed xgettext to work with OO model.
  Item13897: Locale typo - spelled Local

…to Item14152

* commit '4eef383f3c031dce6d2ccba1af7c5657784fc036':
  Item13897: Revert xgettext to the non-OO version in master

…TML formatting.