[FIX] Refactoring BrushRegistryService as Singleton

This circumvents the CF ::requireOnce() call to not return
the cache data (because of require_once()). The brushes
within the cache are successively build within the Singleton
BrushRegistryService.
[FIX] CGLs in ContentController
[FIX] Setting proper brush registry datatype within BushRegistryService
[TASK] Adding brushList marker retrieval within Highlighter Configuration
[TASK] Implementing registered brushes loading
[FIX] PageRenderer hooks only in FE context
[TASK] Removal of brushRegistry cache usage in favor of the Service Singleton
[FIX] Fixing not in an Extbase context: add is_null() check to injectBrushService
[TASK] Introducing dedicated BrushLoader View for registered brush loading

The view also provides a SignalSlotDispatcher slot to manipulate the BrushLoaderView
paths (fx. in order to override the SyntaxHighlighter inline javascript).
[FIX] Fixing addRegisteredBrushes; removal of orphaned getAutoloaderBrushMap
[FIX] Fixing LanguageItems initialization
[FIX] Fixing object initialization in Flexform LanguageItems
[TASK] Removal of ::getBrushListMarker() from Highlighter Configuration
[TASK] Changing brushLabel variable naming in DiscoveryService
[TASK] Removal of unused methods from Highlighter Configuration
[FIX] Re-adding ::hasBrushAlias() for abstract Highlighter Configuration
[TASK] Implementing Brush alias retrieval for Discovery Service through Configuration
[TASK] Fixing some CGLs in Prism + SyntaxHighlighter Configuration classes
[FIX] Fixing ObjectManagerInterface instantiation in BrushDiscovery Service
[FIX] GeneralUtility: cannot instantiate interface fixed in BrushDiscovery