added Git::String

simplifies string conversion between Qt and libgit2
fixed access to invalid memory when converting strings
use Git::Internal::String throughout GitWrap
use Git::Internal::String for CloneOperation as well
removed codec; always convert from and to utf8
inlined toArray() method
use a mutable QByteArray
renamed Git::Internal::String -> Git::Internal::StringHelper
removed private implementations of copy constructor
fixup commit
Try to make Mr. Jenkins a bit more happy...
git_diff_hunk.header is no longer a pointer...

... but an array. So we can use it as before, just not for testing if it points somewhere...
renamed Git::Internal::StrArray -> Git::Internal::StrArayRef
renamed Git::Internal::StrArrayWrapper -> Git::Internal::StrAray
introduced macros for string conversion between Qt and libgit2
replaced StringHelper with GW_StringFromQt/GW_StringToQt macros
removed StringHelper dependency in Internal::Buffer::toString()
removed invalid addresses from Internal::StrArray
removed StringHelper dependency in CheckoutOperation

This urgently needs a suitable test case!
completely removed StringHelper in favor to the GW_StringToQt macros