move QAMQP::Frame to private API

QAMQP::Frame used to be public API mostly for MessageProperties. There
is no longer a need to keep this API public, as it should all be wrapped
with the cleaner Message/Queue/Exchange/Client API set
refactor ValueType

moved ValueType from the QAMQP namespace to QAMQP::MetaType namespace
to avoid clashes with very generic enum strings
refactor Frame::Content to use Message::Property
rename amqp_ files to qamqp

amqp_<filename> is used in at least two other C/C++ projects related
to amqp, so this commit changes all our source to use a qamqp header.
This avoids potential clashes, and is more in line with Qt style