Enhance Simple Image layout controls and overrides

[CySSoO]

Motivation

• Provide parent-level layout controls for the Simple Image prettyblock so non-dev users can choose grid/slider/columns without manually adding classes per item.
• Preserve existing html_class/css_class per-item overrides so developers keep full control when needed.
• Compute layout classes once at parent level and apply them to items unless an item explicitly provides a class, avoiding duplicated logic in the repeater.
• Keep backward compatibility with existing slider behavior and avoid forcing manual classes.

Description

• Added new parent configuration fields in EverblockPrettyBlocks for the Simple Image block: display_mode, columns_desktop, columns_tablet, columns_mobile, and gap to drive default layout behavior from the block settings.
• Updated views/templates/hook/prettyblocks/prettyblock_img.tpl to calculate Bootstrap column classes and gap classes once using the parent settings and to set a layoutItemClass that is applied to each item by default.
• Implemented the override rule where an item with css_class keeps only that manual class while items without css_class get the computed parent classes, and ensured the slider mode uses the parent display_mode and responsive columns for data-items-mobile.
• Kept existing slider options and behavior intact while replacing hard-coded per-item col-* usage with the centralized layout computation.

Testing

• No automated tests were executed for this change.
• Manual inspection: template changes are static and preserve previous conditional visibility and slider logic (no runtime test run requested).

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Codex Task