Breaking: #72310 - EXT:form - Outsource labels and legends to own partials

Description

Labels and legends have been outsourced to their own partials. This step is slightly (but thankfully) breaking.

With this change the duplication of code can be avoided. This helps the integrator to customize the labels/ legends with just one small and central override.

Impact

No deep impact. If an EXT:form template was overridden, it mostly contains the label and/or legend tags and acts like it used to do.

Affected Installations

Any installation using EXT:form since TYPO3 7.5.

Migration

Overridden EXT:form partials could be migrated to use the new central label/ legend partials.

Example changes for Resources/Private/Partials/Default/Show/FlatElements/Checkbox.html.

Old:

<label for="{model.additionalArguments.id}">
    {model.additionalArguments.label}
    <f:if condition="{model.mandatoryValidationMessages}">
        <em><f:for each="{model.mandatoryValidationMessages}" as="mandatoryValidationMessage" iteration="iterator">{mandatoryValidationMessage}<f:if condition="{iterator.isLast}"><f:else> - </f:else></f:if></f:for></em>
    </f:if>
    <f:if condition="{model.validationErrorMessages}">
        <strong><f:for each="{model.validationErrorMessages}" as="errorValidationMessage" iteration="iterator">{errorValidationMessage}<f:if condition="{iterator.isLast}"><f:else> - </f:else></f:if></f:for></strong>
    </f:if>
</label>

New:

{f:render(partial: '{themeName}/Show/AdditionalElements/Label', arguments: {model: model, themeName: themeName})}