AccountListModel: avoid calling getAccountList when unneeded

src/app/MainApplicationWindow.qml

src/app/accountadapter.cpp

src/app/accountlistmodel.cpp

src/app/bannedlistmodel.cpp

src/app/conversationsadapter.cpp

src/app/lrcinstance.cpp

src/app/settingsview/SettingsView.qml

src/app/utilsadapter.cpp

src/libclient/accountmodel.cpp

src/libclient/api/account.h

src/libclient/api/accountmodel.h

src/libclient/authority/storagehelper.cpp

tests/unittests/account_unittest.cpp

tests/unittests/contact_unittest.cpp