Filter module is not required, but is marked as required

Follow-up to: #645468: Filter module no longer needs to be loaded by default

Problem

• Filter module is marked as required = TRUE in its .info file, even though it is not required.

Details

• Filter module was required at some point in the past, since it contained the essential filter_xss() functions. Those have been moved into common.inc though.
• Some modules are calling into check_markup(), but since many/most use-cases have been converted into Text fields, these modules should just simply declare proper dependencies now.
• In general, there are far more modules that depend on Text module's field now.
• The list of callers to check_markup() is a testament of that. :)

Proposed solution

1. Replace the required flag for Filter module with proper dependencies.

Related issues

• #1688162: Replace (most) required modules with proper dependencies