[FEATURE] White/blacklisting allowed CTypes in Column and Area

## Usage examples

In a content/plugin template:

```xml
<flux:grid.row>
	<flux:grid.column>
		<flux:form.content name="content">
			<flux:form.variable name="allowedContentTypes" value="text,textpic" />
		</flux:form.content>
	</flux:grid.column>
</flux:grid.row>
```

In a page template:

```xml
<flux:grid.row>
	<flux:grid.column colPos="1">
		<flux:form.variable name="allowedContentTypes" value="text,textpic" />
	</flux:grid.column>
</flux:grid.row>
```

Alternative syntax:

```xml
<flux:grid.row>
	<flux:grid.column colPos="1"
		variables="{allowedContentTypes: 'text,textpic'}" />
</flux:grid.row>
```