templater: unify "string" and "rawstring"
"rawstring" was introduced by
5ab28a2e9962, but it's no longer necessary
because
c1975809a6b5 and
fd5bc660c9f0 changed the way of processing string
literals.
This patch moves string decoding to the parsing phase as it was before:
('rawstring', s) -> ('string', s)
('string', s) -> ('string', s.decode('string-escape'))