Allow scrolling on fields selector dialog with large number of properties

When configuring search form layout for Page Providers with large number of fields and opening Fields selector, entries overflow past the bottom of the dialog and Cancel/Generate buttons either render off the dialog or are inaccessible.

Steps to reproduce:

1. In Modeler, Configure Page Provider with at least 20+ predicate fields
2. In Designer, configure search form layout for Page Provider
3. In right sidebar, click and drag sub-entry with Page Provider name (under params in Properties section) into code edit area
4. Observe Fields selector dialog at 90-100% browser zoom

Expected behavior: dialog is scrollable to allow configuring each field and Cancel/Generate buttons are selectable/in-dialog

Actual behavior: fields overflow past the bottom; buttons may either render off the dialog or are inaccessible (beyond browser border).

(see attachment on related ticket for reference)