[FEATURE] Option to hide header tags if content empty

TS option: `plugin.tx_fluidcontentcore.settings.header.hideIfEmpty`. Default behavior changed to hide headers if empty, behavior controlled through TS option or directly through template. Field can also be added as Flux form field to control this setting on a per-element basis.
[TASK] Remove schemaLocations, ship XSD

Includes XSD file at Resources/Private/Schemas/FluidcontentCore.xsd