feature(filestore): add elgg_get_file_simple_type() to core api

Consolidate simpletype parsing logic in a new elgg_get_file_simple_type() function added to core api
chore(docs): document simple_type,file hook
deprecate(filestore): deprecate file_get_simple_type() and file_get_general_file_type()

Deprecate file_get_simple_type(), found in mod/file/start.php, and file_get_general_file_type(), found in engine/lib/filestore.php in favour of a pluggable elgg_get_file_simple_type() added to core api