add user search to invite dialog (#1253)

CMakeLists.txt

resources/qml/Root.qml

resources/qml/components/UserListRow.qml

resources/qml/dialogs/InviteDialog.qml

resources/res.qrc

src/InviteesModel.cpp

src/InviteesModel.h

src/MainWindow.cpp

src/UserDirectoryModel.cpp

src/UserDirectoryModel.h

src/UsersModel.cpp

src/UsersModel.h