This adjusts the bundled plugins to do their admin permission checks
based on their admin component's isAccessibleByCurrentUser() method
instead of doing their own isAdmin checks.

Otherwise it's a bit ahrd to decide if access should be granted ;-)

This adds a new method that capsulates the access check that has to be
done to decide if an admin plugin's page should be shown to the user.
The default implementation is the same as before, relying only on the
forAdminOnly() method and the users' isadmin or ismanager status.

Admin plugins themselves can override the method to do additional
checks. In this patch, I added that to the usermanager plugin which will
only return true if the current auth backend can list users.

However the real idea behind this change is that the new method emits a
new event called ADMINPLUGIN_ACCESS_CHECK which would allow plugins to
overwrite it. This way it could be possible to give certain user groups
access to certain admin plugins without giving them admin or manager
permissions.

Note: this does not change how the "Admin" link is shown, it still
depends on ismanager or isadmin. A plugin as mentioned above would need
to influence the display via the MENU_ITEMS_ASSEMBLY event.

Note: this only covers the basic access check. Admin plugins may need
further adjustments for access to other parts of the plugin (like AJAX
components). An additional commit will update this for the bundled
plugins.

Safari 9 in iOS does not support the use of `const` in strict mode. Unfortunately, some of us are stuck with Safari 9 due to not being able to upgrade.

I am a big fan of progressive enhancement, but because all javascript code is packed together and sent off to the client, this small incompatibility breaks all javascript for safari 9 browsers. Switching these two keywords makes everything work again.

There are no other uses of `const` in the codebase as far as I can find. This change has little to no impact on anything else, but slighlty extends the range of supported browsers.

Plugins may want to add further fields to be present when saving drafts.
Plugins may want to execute some js functionality that should be timed to
the saved draft/refreshed lock.

If a plugin does another init() to attach the dw_locktimer to its own editor,
then the default callback would be added a second time, causing unexpected
and undesired behavior.

This includes the changes from the following commits:
6ef45cc1
6ca947f3
0fff419c
87bed8b6

This change is useful for plugins that may want to implement their own toolbar (and thus hide the
default toolbar), but still would like to let the locktimer set the draft status.

Safemode has been removed in PHP 5.4.0. We finally no longer need to
deal with this insanity.

These shouldn't be there anymore since the plugins have been deleted