Added a new field below "Select constant type" to make the number of constants to show selectable

Resolves: forge.typo3.org/issues/66188

This is the error message:
"Warning: htmlspecialchars() expects parameter 1 to be string, array given in /var/www/ay.dev/typo3_src-patched/typo3/sysext/fluid/Classes/ViewHelpers/Form/TextareaViewHelper.php on line 65"

I couldnt reproduce bug where label tag in xml file is marked as array, but this would fix that issue.

Resolves: forge.typo3.org/issues/67142

Larger constant name field for:
- add constant
- rename constant and
- search constant

Resolves: forge.typo3.org/issues/65982

Resolves: forge.typo3.org/issues/66183

* When user edits a file, languages from section 'Select up to two languages' are considered as 'edited languages'.
* If some language is not edited, its file will not be rewritten.

* XLF:
- If user edits default language (language from main file), all localisation files will be rewritten, because default language appears in them as source.

* XML:
- If there is one, merged, xml file, this feature is disabled.

Resolves: forge.typo3.org/issues/67499

Bugfixes

See merge request !4

Resolves: #67421

Resolves: #67864

This exception seems to happen if the last constant was removed/changed, because the line in the AdditionalConfiguration still refers to a file, but this file just contains the xml header then.

Using 'xml:space="preserve"' if constant contains LF or multiple space characters in its source or in its target value.

Related: forge.typo3.org/issues/66178

- removing xml:space tag
- not replacing newline with br tag

Updating LFEditor xlf files:
- removing xml:space tag
- replacing br tag with newline

Resolves: forge.typo3.org/issues/66178

Related: forge.typo3.org/issues/66178

For reference languages are chosen only translated languages.

Related: forge.typo3.org/issues/65631

EN doesn't need to be specified in configuration as viewed language.