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: Do not attempt to run abstract test classes

There is no reason to try to run test cases of a class that is abstract.  By
skipping them we allow developers to create abstract test case classes that
have test functions in them.  This is especially helpful when someone is
testing multiple implementations of the same service interface.  Most of their
tests can be in the abstract class, and then they can create concrete test
classes for each of their implementations and inherit all of the testing that
is built into the abstract class.
Filter notice styling and panel adjustments

* Based on @clarkepaul's commit here:
https://github.com/silverstripe-droptables/sapphire/commit/e5de6c06489f3
449ee8131bf4ee2a4e1b2380bee
* Kept margin adjustments, but avoided implementing tab changes, as
tabs have chnaged a lot since original commit
Batch actions styling

* Removed cog icon (usually associated with settings), and tidied up
padding (based on @clarkepaul's commit:
https://github.com/silverstripe-droptables/sapphire/commit/b09cd5f7f8e73
4df1f7ca0d387a5b97a03a3b604)
Preview Background styling

* Added background color to preview for sites that don't have a
background color (based on @clarkepaul's commit:
https://github.com/silverstripe-droptables/sapphire/commit/7750c8f75fc4a
764b384c6e19dbe543c4811cdcb)
Merge pull request #2058 from adrexia/filter-panel

Filter notice styling and panel adjustments
Merge pull request #2060 from adrexia/preview

Preview Background styling
Merge pull request #2059 from adrexia/batch-actions

Batch actions styling
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.
Behat: New fixture notation in "manage files" tests
Behat: Support for TreeDropdownField
Behat fixture refactoring (shared factory instance)
Behat: Copy fixture identifier to "Name" or "Title" by default

This makes fixture setup less verbose, particularly for behat
tests (e.g. with "Given a page 'About Us'")
Behat: Use explicit fixtures
Merge remote-tracking branch 'origin/3.0' into 3.1

Conflicts:
	tests/behat/features/bootstrap/SilverStripe/Framework/Test/Behaviour/CmsUiContext.php
Enhancement: replaced privete with protected in order to have MySQLDatabase class extensible
Behat: Fixed step notation