[FEATURE] allow selection of categories pid

[Teisi]

Problem / Motivation

The plugin FlexForm field settings.flexform.selectCategory (Configuration/FlexForm/Flexform.xml) currently lists all category records (tx_jpfaq_domain_model_category) of the entire installation:

<config>
    <type>select</type>
    <renderType>selectMultipleSideBySide</renderType>
    <foreign_table>tx_jpfaq_domain_model_category</foreign_table>
    <foreign_table_where>ORDER BY category</foreign_table_where>
    ...
</config>

This is problematic in multi-site / multi-domain installations:

1. Categories cannot be scoped per site or page tree. Editors working on site A see (and can select) the FAQ categories of site B and C as well. With several sites or several independent FAQ trees within one site, the select box becomes cluttered and error-prone.
2. All localizations are listed as separate entries. FormEngine does not filter foreign_table selects by language, so every translated category record shows up alongside its default-language original, further bloating the list.

Proposed solution

Enhance the foreign_table_where of settings.flexform.selectCategory so that integrators can optionally restrict the selectable categories via Page TSconfig, and so that the list is filtered by the language of the content element:

<foreign_table_where>
    AND (0 IN (###PAGE_TSCONFIG_IDLIST###) OR {#tx_jpfaq_domain_model_category}.{#pid} IN (###PAGE_TSCONFIG_IDLIST###))
    AND {#tx_jpfaq_domain_model_category}.{#sys_language_uid} IN (-1, ###REC_FIELD_sys_language_uid###)
    ORDER BY category
</foreign_table_where>

How the PID restriction works (opt-in via Page TSconfig)

TCEFORM.tt_content.pi_flexform.jpfaq_faq.main.settings\.flexform\.selectCategory.PAGE_TSCONFIG_IDLIST = 42,43

Page TSconfig	Resulting behavior
(not set)	Marker defaults to 0, so 0 IN (0) matches → all categories are listed (current behavior, fully backwards compatible)
PAGE_TSCONFIG_IDLIST = 42,43	Only categories stored on pid 42 or 43 are listed
PAGE_TSCONFIG_IDLIST = 0	Explicitly resets to "all" (useful to undo an inherited restriction within a sub tree)

Since Page TSconfig is inherited down the page tree, this supports per-site defaults and per-branch overrides — e.g. several independent FAQ trees within a single site, each with its own category storage folder.

How the language filter works

###REC_FIELD_sys_language_uid### resolves to the language of the content element the plugin lives in (in FlexForm context, FormEngine reads the marker value from flexParentDatabaseRow.

• Plugin in default language → sys_language_uid IN (-1, 0): default-language categories plus "all languages" records
• Translated plugin (e.g. sys_language_uid = 1) → sys_language_uid IN (-1, 1): only categories translated into that language

Backwards compatibility

• The PID restriction is fully opt-in: without any Page TSconfig the list stays unrestricted, exactly as today.
• The language condition does change current behavior insofar as localized category records are no longer offered for default-language plugins (and vice versa). Arguably the current behavior — mixing all languages in one list — is more of a bug than a feature.