Fix menu_pool teardown after exception throwing another exception.

This conceals the original error that you actually want to fix:

	ERROR at teardown of ResourceDetailAppTests.test_left_sidebar_contains_common_metadata
	.ve/local/lib/python2.7/site-packages/django/test/testcases.py:187: in __call__
	    self._post_teardown()
	.ve/src/django-harness/django_harness/override_settings.py:48: in _post_teardown
	    original_post_teardown(innerself)
	.ve/src/django-cms/cms/test_utils/testcases.py:98: in _post_teardown
	    menu_pool.clear()
	.ve/src/django-cms/menus/menu_pool.py:97: in clear
	    cache_keys.delete()
	.ve/local/lib/python2.7/site-packages/django/db/models/query.py:467: in delete
	    collector.collect(del_query)
	.ve/local/lib/python2.7/site-packages/django/db/models/deletion.py:166: in collect
	    reverse_dependency=reverse_dependency)
	.ve/local/lib/python2.7/site-packages/django/db/models/deletion.py:77: in add
	    if not objs:
	.ve/local/lib/python2.7/site-packages/django/db/models/query.py:100: in __nonzero__
	    self._fetch_all()
	.ve/local/lib/python2.7/site-packages/django/db/models/query.py:857: in _fetch_all
	    self._result_cache = list(self.iterator())
	.ve/local/lib/python2.7/site-packages/django/db/models/query.py:220: in iterator
	    for row in compiler.results_iter():
	.ve/local/lib/python2.7/site-packages/django/db/models/sql/compiler.py:713: in results_iter
	    for rows in self.execute_sql(MULTI):
	.ve/local/lib/python2.7/site-packages/django/db/models/sql/compiler.py:786: in execute_sql
	    cursor.execute(sql, params)
	.ve/local/lib/python2.7/site-packages/django/db/backends/util.py:47: in execute
	    self.db.validate_no_broken_transaction()
	.ve/local/lib/python2.7/site-packages/django/db/backends/__init__.py:372: in validate_no_broken_transaction
	    "An error occurred in the current transaction. You can't "
	E   TransactionManagementError: An error occurred in the current transaction. You can't execute queries until the end of the 'atomic' block.

Conflicts:

	cms/test_utils/testcases.py
Send custom events when the structureboard is shown and hidden.

This allows websites to adapt by changing page styles for example, so that
all placeholders are visible. This is useful for rotators and other features
that hide parts of the page or place them offscreen.
Reduce interval timer frequency of javascript layout updates.

Stops the structure view from burning 100% CPU.