Fix multiline string property value parsing

[dcronqvist]

Description

All string properties for the .tmx file format expected the string value to exist in the value attribute. However, for multiline string properties, the string value will be placed inbetween the property tags (body). This is addressed in this PR so that we properly parse multiline string properties without causing a crash.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist

• Tests have been added/updated to cover new functionality.
• Documentation has been updated for all new changes (e.g., usage examples, API documentation).