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

Limits possible selections in the new content element wizard based on parameters defined in the Column or Area (or both) into which content is being inserted.

## 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>
```

Using PHP (Flux Form Components):

```php
$grid = \FluidTYPO3\Flux\Grid::create();
$row = $grid->createContainer('Row');
$column = $row->createContainer('Column');
$column->setColumnPosition(1)
	->setVariable('allowedContentTypes', 'text,image');
```

To blacklist, use `deniedContentTypes` instead of `allowedContentTypes` as variable name. Both can be used together: first, the whitelist is applied if it has any types contained, and second, the blacklist is applied to remove elements from the list of allowed types (which is either the full list or the previously whitelist-filtered version, if a whitelist was specified).

Fixes: #197
[TASK] Restore TCA and expose fields to select relationship
[BUGFIX] Remove one of twice declared inherited interface method

Fixes: #450
[BUGFIX] Only clear cache when "Clear configuration cache" is pressed
Merge pull request #447 from NamelessCoder/allowedcontent

[FEATURE] White/blacklisting allowed CTypes in Column and Area
[TASK] Rename an argument to fit actual contents
[BUGFIX] Use extension key, not qualified name, for isLoaded check

Fixes: #429
[TASK] Include vendor name in exposed view preparation
Merge pull request #455 from NamelessCoder/settingsscope

[TASK] Improve Request settings for ExposedTemplateView preparation
[BUGFIX] Fix SQL error in BackendConfigurationManager

The first argument for exec_SELECTcountRows was missing.
[BUGFIX] Fix SQL Injection possibilities

Table name is fetched from GET parameters and
passed directly to a database query.

This change checks the table name against a whitelist
(TCA) and throws an exception if the table name does
not exist.

Additionally another value passed by GET is sanitized
to integer before using in SQL string
Merge pull request #458 from helhum/sqli-fix

Fix BackendConfigurationManager
[BUGFIX] Class not found error on 6.1
[TASK] Fix an incorrect package annotation value
[TASK] Update object type in return annotation.

Current annotation is not recoganise by the IDE. It must be either
prepended by a "\" or simply the class alias.
Merge pull request #464 from fudriot/task-code-clean-up

[TASK] Update object type in return annotation.
[BUGFIX] Add condition before converting FlexForm to Array

There is a special case, when saving a new record in the BE,
variable $row[$fieldName] is already an array and must not
be processed by the configuration service. Otherwise, a
warning is raised, telling:

PHP Warning: substr() expects parameter 1 to be string, array given.
Merge pull request #463 from fudriot/bugfix-conditional-flexform-conversion

[BUGFIX] Add condition before converting FlexForm to Array
[BUGFIX] Fixed pid 0 problem by copy elements

If you copy content elements, the parent elements are always set on pid 0.
[BUGFIX] Remove doubly declared abstract functions.

The interface "MultiValueFieldInterface" already declares some
functions that are redeclared in its child interface
"RelationFieldInterface". This is causes a fatal error in older PHP
versions.