Maintenance

[TASK] code quality: - invalid usage of deprecated Event/PerformancesViewHelper removed from templates - early return removed from render method (Dirk Wenzel)
[TASK] code cleanup (Dirk Wenzel)
[BUGFIX] CI: sonar-section readded. TYPO3 version 9 removed from matrix (Dirk Wenzel)
[TASK] require typo3/cms:8.7.x (next version will not support TYPO3 9 yet) (Dirk Wenzel)
[BUGFIX] prevent error caused by missing section in template file (Dirk Wenzel)
[TASK] invalid and deprecated method calls and configuration options replaced (Dirk Wenzel)
[TASK] deprecated classes and functions replaced - extbase ArrayUtility by core ArrayUtility - GeneralUtility::requireOnce by require_once (Dirk Wenzel)
[TASK] fixed unit tests - respect strict argument types of count() method (Dirk Wenzel)
[TASK] fix unit tests - obsolete coverage annotation removed - make sure that 'findDemanded' method returns a mock QueryResult (Dirk Wenzel)
[TASK] require typo3/cms-core instead of typo3/cms and use typo3/minimal for travis ci. (Dirk Wenzel)
[TASK] View/ConfigurableViewInterface and Controller/Backend/BackendViewTrait implemented. (Dirk Wenzel)
[TASK] Unit Tests refactored for current version of testing framework. - DemandedRepositoryTrait->generateQuery is now public (Dirk Wenzel)
[TASK] drop support for TYPO3 version 7.x (Dirk Wenzel)
[BUGFIX] CI remove build folder before sonar scan. (Dirk Wenzel)
[TASK] travis CI: sonar-runner coverage file paths configured, generate junit style log for phpunit (Dirk Wenzel)
[TASK] CI configuration for sonar changed: use identifier typescript in properties (Dirk Wenzel)
[TASK] CI exclude TypoScript folder from TypeScript sonar check (Dirk Wenzel)
[TASK] code quality

• deprecated '' tag in HTML templates replaced
• jQuery library removed
• duplicate markup moved to partial (Dirk Wenzel)
  [TASK] CI: ignore duplication in ext_tables and TCA files. (Dirk Wenzel)
  [TASK] travis CI: enable some more branches for sonarcloud (Dirk Wenzel)
  [TASK] travis CI: call sonar-runner with coverage. (Dirk Wenzel)