#381346 by ayalon: Allow Taxonomy: Tid as a field, which is now needed for rewrites which may need tids.

Fix taxonomy term ID field, filter and argument for term views, which were incorrectly keying off of term_node.

Major re-organization of handlers. PLEASE NOTE: This drastically affected the Views module API and until modules are updated to match, they will stop working. Efforts have been made to ensure that this won't cause your site to crash, but that's partly up to the individual module as well.

Create a "soft" fail condition for arguments, which means that if an argument is part of a % placeholder in a URL, failing validation is not actually a failure IF there is some other action to take when validation fails.

Exposed filter memory was being trashed by a rewriting validate; also, exposed filters that decided not to participate weren't properly storing that status in session. This fixes both things.

Exposed filters needed to use $form_state[input] not $form[#post], which was causing the "remember" setting to fail intermittently; also, if filters are not overridden use the default display ID for session storage so that remembered filters can be shared across displays.

Modify $view to only use handlers for the field items, since it cares a lot more about the handlers than it cares about the actual storage of the field items. This could have implications for modules, particularly date and calendar.

Some refactoring of how handlers are handled, particularly with their default options. This is an unfortunately widespread change and will affect modules utilizing the API.

Provide new "reduce list" option which lets any in_operator based filter show only options selected by the administrator as an exposed filter.

#259721: Ensure "date" filters do not affect the query when left empty and set to optional. Also ensure they get validated on exposed input when not set to optional.

#259914: Exposed user: name filter broken. This also affected taxonomy, so fixed that at the same time.

A special argument that will modify the depth of the previous term argument, so we can emulate the original taxonomy/term URL.