Rework keyword parser rule
A keyword is either a plain identifier, or two identifiers concatenated
through a colon. Make this a bit clearer, aiding debugging of issues.
Also push KeywordContext down to
StatementContextVisitor.getValidStatementDefinition(), so that we can
improve efficiency once IDENTIFIER token has been fixed up.
Change-Id: Ic64285255564ae33f8ad208b9a3598ba20cd26fb
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit
220f54de00be7e3906349059fdd4bb139859ee23)