List table: Select %s shouldn't be a row header

[afercia]

See related #31654 and #26167.

In all the List Tables in the admin, the "Select %s" (where %s is the name of the Post, Attachment, Plugin, User, etc,) is used as row header:

<th scope="row" class="check-column">

This is not correct from a semantics and accessibility point of view for all the reasons mentioned in #31654. The row header should be the cell with the main object the table refers to: the Post title, the Plugin name, etc.

We could also consider to use the abbr attribute: ​http://www.w3.org/TR/html5/tabular-data.html#attr-th-abbr
that would be consistent with what is currently proposed in sortable column headers ticket

[afercia]

One more good reason to address this issue is that, when a post is locked and the lock icon appears, the icon has no label or text that can be announced so screen readers will read out the column header "Select All";