Work around startup UI freeze caused by QtTextToSpeech

This merge request works around an annoying startup hang that I introduced when adding text-to-speech to NeoChat. The previous implementation was a QML singleton that used the `TextToSpeech` QML component. Unfortunately that component blocks the UI thread when first loading it, while it connects to speech-dispatcher.

This MR just rewrites that singleton in C++, and moves initialization of QtTextToSpeech to the first time you read a message aloud. It doesn't fix the performance problem, but it at least stops it from affecting startup.

In the future, I'd like to move speech operations to a background thread to completely mitigate the initialization freeze.

src/app/CMakeLists.txt

@@ -37,10 +37,8 @@ add_library(neochat STATIC
     identityserverhelper.h
     models/commonroomsmodel.cpp
     models/commonroomsmodel.h
-)
-
-set_source_files_properties(qml/TextToSpeechWrapper.qml PROPERTIES
-    QT_QML_SINGLETON_TYPE TRUE
+    texttospeechhelper.h
+    texttospeechhelper.cpp
 )
 
 set_source_files_properties(qml/OsmLocationPlugin.qml PROPERTIES
@@ -87,7 +85,6 @@ ecm_add_qml_module(neochat URI org.kde.neochat GENERATE_PLUGIN_SOURCE
         qml/InvitationView.qml
         qml/AvatarTabButton.qml
         qml/OsmLocationPlugin.qml
-        qml/TextToSpeechWrapper.qml
         qml/FullScreenMap.qml
         qml/LocationsPage.qml
         qml/LocationMapItem.qml
@@ -204,6 +201,7 @@ target_link_libraries(neochat PUBLIC
     Qt::Multimedia
     Qt::Network
     Qt::QuickControls2
+    Qt::TextToSpeech
     KF6::I18n
     KF6::Kirigami
     KF6::Notifications