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 consolidated elgg_file_get_simple_type() added to core
feature(filestore): add elgg_file_get_simple_type() to core API

Consolidate simpletype parsing logic in a new elgg_file_get_simple_type() function added to core API
chore(docs): document the use of simple_type,file plugin hook

Documents the simple_type,file plugin hook and adds some use case information
chore(file): clean up and make use of the new simple type api

Clean up and adjust file plugin to the new elgg_file_get_simple_type() api