Added allocation constructor (makes it easy to convert right into a buffer

Added allocation constructor (makes it easy to convert right into a buffer
allocated to size by this class), copy constructor (egcs does not like
const wxCharBuffer& mb_str() { return wxCharBuffer(...); }, so it HAS to be
copiable), and assignment constructor (so it can be stored to a variable
for efficiency, instead of being converted every time the string is needed).

(The copy and assignment operator wipes the source, so the buffer is
essentially passed on to the new instance without being freed by the old.)

wcsdup() is not declared in glibc by default, so use malloc() instead.
Added some 'convenience' macros.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@2107 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775