Restrict layout blocks scope to forms only

Layout blocks are meant to be used for reusable parts of layouts (manually created by users or migrated from JSF).

Currently in Studio they include any kind of custom element, including very technical ones that should not actually be in the listing.

=> that's a problem for everyone because they don't understand the purpose of the editor anymore, especially considered that we want to add more of them in the future for other areas (like table blocks to reuse columns for folderish docs / documents listings, equivalent of table layouts in JSF UI).

AC

• Layout blocks only show elements from the "forms" folder
• New form elements are created in the "forms" folder
• For existing elements, migration needs to be handled manually