support contexted lines in read_strings_po
fixed to use lists correctly
implemented (untested) contexts for inplace strings
Implemented contexts for moved strings (untested)
use context for pointers, not for occurances
Treat uncontexted line as context=0

Because POEditor omits msgctxt=0
typo
fixes
don't try to replace contexted values inplace
don't remove translated contexted key extra times
support multiple contexts for one item

with comma-separated
Only append range and remove key when all ctxt are translated
don't duplicate identical strings for different ctxts
really don't create unneeded entities (fix previous commit)
Merge branch 'contexts'

Implemented support of contexts (in PO files only). It allows to
distinguish between several identical string.