UI fatal caused by views argument handlers no longer can provide their own default argument handling

Problem

Some argument handlers provide their own default argument handling, like the Date argument. The Date argument handler does so by extending the list of of options in the defaultArgumentForm. An exception is however thrown on saving the form.

Steps to reproduce:

• Create a new view /admin/structure/views/add using (node) content
• Under advanced tab, add new Contextual filter, select Created date (node.created)
• On "When the filter value is not available", select "Provide default value", on the drop-down select "Current date".
• Try to submit this form.

Result: A fatal error occurred on dblog: Drupal\Component\Plugin\Exception\PluginNotFoundException: The "date" plugin does not exist.

Proposed resolution

Get the date handlers in line with the other default argument handlers.
Add a 'Current date', a "Current node's creation time" and "Current node's update time" default argument handler.

Beta phase evaluation

Manual Test Results

See #68.

To-Do Tasks

Determine whether or not hook_update_N() is necessary and writing it.