The Ctrl+Q shortcut is assigned to Quit action in most applications on
GNU/Linux. Command+Q is used on macOS. The added shortcut should be
automatically mapped to Command+Q on macOS judging by the following
quote from QKeySequence class documentation:
Note: On macOS, references to "Ctrl", Qt::CTRL, Qt::Key_Control and
Qt::ControlModifier correspond to the Command keys on the Macintosh
keyboard
QKeySequence::Quit could be used as the default key sequence in place of
`Qt::CTRL | Qt::Key_Q`. This would leave the shortcut unassigned by
default on Windows. But YACReader doesn't use QKeySequence::StandardKey
anywhere, so perhaps this shortcut should be hard-coded too.
The shortcut is particularly useful when Close to tray option is
enabled, because in this case closing the Library window with a system
window manager shortcut simply hides it.
