serializable: make serialize const

src/account.cpp

src/account.h

src/config/serializable.h

src/jamidht/jamiaccount.cpp

src/jamidht/jamiaccount.h

src/media/video/video_device_monitor.cpp

src/media/video/video_device_monitor.h

src/preferences.cpp

src/preferences.h

src/sip/sipaccount.cpp

src/sip/sipaccount.h

src/sip/sipaccountbase.cpp

src/sip/sipaccountbase.h