accountcombobox: show unread count

src/app/accountadapter.cpp

src/app/accountadapter.h

src/app/accountlistmodel.cpp

src/app/accountlistmodel.h

src/app/conversationsadapter.cpp

src/app/mainview/components/AccountItemDelegate.qml

src/app/qmlregister.cpp

src/app/systemtray.cpp

src/app/systemtray.h

tests/unittests/globaltestenvironment.h