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'))