Allow String Filter to be Attached to ON or WHERE clause

Problem/Motivation

The currentString Filter in Views is limited to being applied to the WHERE clause, which effectively renders the "OUTER" join as an "INNER" join once a condition is applied to any joined table. There are many cases where it is desirable to apply conditions to a related field, but to also show a NULL record if no relation exists, that is, to set conditions on the ON clause as opposed to the WHERE clause.

Proposed resolution

Add an option to the configuration of Field Comparison Filters to select the target of the filter condition, either "WHERE" or "ON". Expand class "views_handler_filter_string" to add the join "extra" if "ON" is selected. If "WHERE" is selected, use default "->add_where_expression()".
Make WHERE as the default to preserve behavior of all existing views.

Remaining tasks

• Initial testing
• Add support for query location in all comparison types in views_handler_filter_string.inc (eg op_starts(), op_ends(), ...):
  • op_equal
  • op_contains
  • op_word
  • op_starts
  • op_not_starts
  • op_ends
  • op_not_ends
  • op_not
  • op_shorter
  • op_longer
  • op_regex
  • op_empty
• Modify field selector to prevent base_table properties to be used as RIGHT fields when ON is selected.
• Insure that filter order insures that fields are not used in an ON clause before their table is added.
• Add test bot tests
• Wider testing by community.
• Insure compatibility with Import/Export functionality to migrate Views among installs.

User interface changes

Add drop-down selector for "Condition Location", with options "WHERE" (default) and "ON".

API changes

None.

Data model changes

The addition of a single configuration option "op_location" for Views that use this filter.