* refactor: add libtransmission-app
* refactor: add libtransmission-app/display-modes.h
* refactor: use app::SortMode, app:ShowMode in Qt client
* feat: add to_variant(), to_value() in serializer
* refactor: use app::SortMode in GTK client
* refactor: use app::ShowMode in GTK client
* refactor: make naming consistent with libtransmission-app
* refactor: remove Session::addKeyName()
refactor: remove Session::removeKeyName()
these should have been removed a long time ago
* fix: better logging on error in Converters
* refactor: add Qt-app-specific variant converters
* refactor: use tr_variant::unmanaged_string(tr_quark) when in torrent_get table mode
Support special (optional) `--` argument to explicitly separate options
from filenames. Support special `---` argument to separate client
arguments from Qt arguments.