Dialog - optional modal mode

[BillBuilt]

This adds a new dialog.ContentProps field: ShowModal *bool (default true) to allow disabling of the dialog element's backdrop in cases where it needs to be hidden (example: it covers other popup elements used by the dialog's contents)

Closes: #541

[axadrn]

Thanks! Pushed a follow-up commit: replaced ShowModal *bool with DisableModal bool to keep it consistent with the existing DisableClickAway/DisableESC API, removed the *bool pointer workaround in the showcase, fixed the missing TOC entry, and registered the showcase in render-showcases.