Remove jQuery dependency

[nikosdion]

As far as I can tell, it's currently only used for AJAX in the backend Item view.

We may also have to change any graphs to the same library we use in Admin Tools and Akeeba Subscriptions.

Unfortunately, this means I have to pull in the entire Akeeba.System and Akeeba.Ajax libraries from Akeeba Backup.

I normally wouldn't bother if it wasn't for the fact that I can no longer depend on Joomla shipping jQuery.