CLI subcommand for link creation

[kirillt]

The app should gain CLI flag for running link creation dialog directly, skipping list of existing links:

ark-shelf --add-link

This way it would be possible to bind a hotkey easily for this operation which should be quite common.

Question: what to do if there is already running process of ARK Shelf? Ideally, we should re-use open window.

Extra: we could design the sub-command such a way it would convenient to use it without GUI at all.