Moved checkboxset  option list generation to separate method

This allows subclasses and extensions time to modify the list of options and their configuration without having to override the entire Field method.

A more flexible way to implement silverstripe#3311
Merge pull request #3319 from wilr/movetogetoptions

Moved option list generation to separate method
CSS: Fix upload field display for preview open
BUG: CMS tree filters doesn't count the correct number of children for deleted pages

This is a bug that combines Hierarchy, Versioned and LeftAndMain admins and CMSSiteTreeFilters.

This bug can be reproduced by having a large site tree with enough deleted pages in it so it doesn't
pre load all the children pages when initially opening an admin. Filter by either 'All pages including deleted'
or 'Deleted pages'. For CMS users it will look like deleted pages are gone.

The solution involves a couple of smaller fixes in both CMS and framework modules.

1) Ensure that 'numHistoricalChildren' are used instead of 'numChildren' when dealing with deleted pages
2) LeftAndMain::currentPage() deletes all the 'marking' cache previously built up by Hierarchy::markPartialTree()
3) Use Versioned::get_included_deleted() instead of raw DB queries against the DataObject tables when calculating parents in CMSSiteTreeFilter
Merge pull request #3309 from stojg/issue/cms-1049

Delete parent page, child pages cannot be found anymore
Merge pull request #3329 from adrexia/upload-with-preview-fix

Fix upload field display for preview open
FIX TemplateManifest prevent cache collision
Removed link to outdated docs mailinglist
BUG Fix periodic tinymce layout refresh
Merge pull request #3335 from tractorcow/pulls/3.1/tinymce-self-cleanup

BUG Fix periodic tinymce layout refresh
API Let update interval of tinymce be changed or disabled