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
chore(cleanup): simplified docs and tightened code style
chore(refactoring): cleans up and simplifies logic in filestore lib
feature(messages): improved UI for messages listing

The checkbox for bulk actions was moved to the beginning of the row and
an excerpt of the message is shown in the listing

fixes: #4739
Merge pull request #7352 from mrclay/hypeJunction-simpletype

Adds elgg_get_file_simple_type() to core
Merge pull request #7343 from jeabakker/messages-checkbox-layout

feature(messages): improved UI for messages listing