Add persistentData to store dc-fw-version

Reading dc-fw-version is somehow complicated ...
Id does not work reliable on startup, so we do read it also on every
diagRequest().
Version string is then stored in persistent data.
This data can be used e.g. by other tools to show the
device-controller-firmware-version.

DCPlugin.pro

@@ -80,7 +80,8 @@ HEADERS += \
     src/ATBAPP/support/CashUtils.h \
     src/ATBAPP/support/DBusControllerInterface.h \
     src/ATBAPP/support/JSON.h \
-    src/ATBAPP/support/PTUSystem.h
+    src/ATBAPP/support/PTUSystem.h \
+    src/ATBAPP/support/PersistentData.h
 
 SOURCES += \
     src/ATBAPP/ATBHealthEvent.cpp \
@@ -91,7 +92,8 @@ SOURCES += \
     src/ATBAPP/support/CashUtils.cpp \
     src/ATBAPP/support/DBusControllerInterface.cpp \
     src/ATBAPP/support/JSON.cpp \
-    src/ATBAPP/support/PTUSystem.cpp
+    src/ATBAPP/support/PTUSystem.cpp \
+    src/ATBAPP/support/PersistentData.cpp
 
 DISTFILES += \
     generate-version.sh