NEW: Add Thumbnails to 'Choose another file' dialogue

Very difficult to choose an image based on file name alone. Added thumbnail and reduced rows from 10 to 8 to compensate for increased height of rows with thumbnail included.
Fix: Enable delete button for users with canDelete() but no canEdit() permissions

$form->makeReadonly(); iterates over all fields/actions, disabling the 'delete' button even when the user has permission
BUG Trigger TinyMCE onChange more frequently

Periodically check for inline changes when focused,
since TinyMCE's onChange only fires on certain actions
like inserting a new paragraph, as opposed to any user input.
This also works around an issue where the "save" button
wouldn't trigger if the click is the cause of a "blur" event
after an (undetected) inline change. This "blur" causes onChange
to trigger, which will change the button markup to show "alternative" styles,
effectively cancelling the original click event.
BUG Correct relation saving in DataObject->duplicateRelations()

This caused problems when duplicate() was used in the CMS UI
to duplicate a SiteTree object. Since every object of this type
has a ParentID relation, it copied this empty relation into
new "ghost page".

See https://github.com/silverstripe/silverstripe-cms/issues/689
ENHANCEMENT: allow disabling of Versioned prepopulate_versionnumber_cache

In large sites this can take a very long time, drastically slowing down the CMS
admin.  Even though the versions will then need to be queried individually,
this is still significantly faster than loading hundreds of thousands of
version numbers in one query and populating the cache array.
Allow PasswordValidator to be translated
Merge pull request #2076 from halkyon/passwordvalidator_translate

Allow PasswordValidator to be translated
BUG If BackURL set, validation errors send the user to wrong place.

If there's validation errors in the ChangePasswordForm, the user
is taken to the BackURL because redirectBack() will go there if
it's set.

Instead of this, just redirect back to the "changepassword" action
on the Security controller.
Update documentation static declarations to private

Also spelling, grammar and line length clean up.
Merge pull request #2077 from halkyon/changepassword_validation_redirection_issue

BUG If BackURL set, validation errors send the user to wrong place.
Merge pull request #2052 from chillu/pulls/onchange-tinymce-trigger

BUG Trigger TinyMCE onChange more frequently
Merge pull request #2054 from chillu/pulls/dataobject-duplicate-hasone

BUG Correct relation saving in DataObject->duplicateRelations()
BUG Temporarily disable overwrite warning on HtmlEditorField "insert media" dialogue until this feature properly supports validation error recovery.
Merge pull request #2044 from jonom/patch-3

FIX #527 : Add Thumbnails to 'Choose another file' dialogue
Merge pull request #2080 from tractorcow/3.1-mediadialog-overwrite-hack

BUG Temporarily disable overwrite warning on HtmlEditorField "insert media".
Add autocomplete=off switch for the password field.

Some clients require disabling of the browser password handling
mechanisms. Add a switch to make it possible without hacking the core.

No change to default behaviour.
Merge pull request #2085 from mateusz/password-autocomplete

Add autocomplete=off switch for the password field.
BUG Fix JS issues in treedropdownfield for link editor
Merge pull request #2086 from robert-h-curry/1954-link-editor

BUG Fix JS issues in treedropdownfield for link editor
Merge pull request #2062 from jthomerson/enhancement_3.1_allow_disabling_prepopulate_version_number_cache

ENHANCEMENT: allow disabling of Versioned prepopulate_versionnumber_cache