(edit_read_syntax_rules): fix pointer tests.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
(edit_read_syntax_rules): minor refactoring of keyword first chars collect.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
mceditor: syntax: reimplement keyword list using GPtrArray.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
mceditor: syntax: remove syntax_keyword_t::first member.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
mceditor: syntax: reimplement context list using GPtrArray.