Add an 'on_block_load' event.

Re-worked pull request 589 that I made on the master branch into
the 553 devel branch.
https://github.com/concrete5/concrete5/pull/589

To avoid performance impact where it is not needed, firing the event is
controlled by ENABLE_ON_BLOCK_LOAD_EVENT definition. If
ENABLE_ON_BLOCK_LOAD_EVENT is set, 'on_block_load' will be fired. Event
handlers have the opportunity to return a modified block record.

This will facilitate adjusting loaded block data. For example, it is now
possible to develop an attribute or package to modify the file set
loaded by a gallery depending on the page or user. We can make stacks
and global areas responsive to where they are shown, who is logged in,
or who owns the page.
Revert "Fix security gap between administrators and super user"

This reverts commit b59c5df836b1eeabb9761d53cf4b10e51affc5f8.
replaced view.css with addHeaderItem to avoid unnecessary request
fixed
Merge pull request #598 from JohntheFish/5.5.3-devel-blockload

Add an 'on_block_load' event.
updating block events
Merge branch '5.5.3-devel'
Update web/concrete/blocks/slideshow/view.css

as mentioned here http://www.concrete5.org/community/forums/customizing_c5/tiny-fix-for-slideshow-block/
Merge pull request #625 from Remo/patch-11

remove underscore due to a-tag in slideshow
Merge branch 'master' of github.com:concrete5/concrete5
backward compatibility with bootstrap 1 on dashboard pages span/offset. upgrade fixes
rewrote next/previous block to work better from a performance standpoint
automatically approving updates to pages and themes when initiated from sitemap
maintaining unicode on upgrade
page search tweak
dashboard tweaks
layout fixes
footer required
global area bug fix
close on core theme