Problem: Cannot use "g<" mapping with ext_messages. Mapping displays
the scrollback buffer since the last command, but the
scrollback buffer is not populated with ext_messages.
Solution: With ext_messages; store messages in the history that otherwise
wouldn't be. Mark them as temporary messages to be deleted when
the scrollback buffer would be cleared. To this end, make the
message history a doubly-linked list such that messages can be
removed from an arbitrary position.
Outlook: Default ext_messages UI might not show the hit-enter prompt
so we want "g<" to work as a recommended way to show messages
for the last command (prompted by an indicator).
Problem: Highlight group id is not propagated to the end of the message call
stack, where ext_messages are emitted.
Solution: Refactor message functions to pass along highlight group id
instead of attr id.
Remove `export` pramgas from defs headers as it causes IWYU to believe
that the definitions from the defs headers comes from main header, which
is not what we really want.
It isn't really useful to put anonymous enums only used as arguments to
functions calls in _defs.h headers, as they will only be used by a file
that calls those functions, which requires including a non-defs header.
Also move os_msg() and os_errmsg() back to message.h, as on Windows they
are actual functions instead of macros.
Also remove gettext.h and globals.h from private/helpers.h.
