accountcombobox: use QSortFilterProxyModel to filter out current account

src/accountadapter.cpp

src/accountadapter.h

src/accountlistmodel.cpp

src/accountlistmodel.h

src/mainview/MainView.qml

src/mainview/components/AccountComboBox.qml

src/mainview/components/AccountComboBoxPopup.qml

src/mainview/components/AccountItemDelegate.qml

src/qmlregister.cpp