lib: fully detach CTF IR and CTF writer implementations

lib: fully detach CTF IR and CTF writer implementations

This patch makes the CTF IR and CTF writer implementations completely
independent. To achieve this, files were partly or fully copied from CTF
IR to CTF writer directories, and everything part of the copied "common"
part was prefixed with `bt_ctf_` or `BT_CTF_` instead of `bt_` or `BT_`.

For the moment, both CTF IR and CTF writer implementations have their
own common part, each of which is only common to its own implementation
now.

Having independent implementations makes it easier to modify one,
sometimes drastically, without caring about not changing the common part
too much.

The BT_LIB_LOG*() macros do not accept the `w` category anymore, making
the CTF writer objects not supported by them. The `_` category is still
accepted for the common part of the CTF IR objects.

The CTF writer clock class API is hidden instead of being public as of
this patch because this was faster to implement and it does not break
backward compatibility with Babeltrace 1.

For some reason, `plugins/text/pretty/print.c` used some `BT_CTF_*`
enumeration members, so this is fixed too.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>