[desktop] make toggle option

[undg]

To quickly invoke it with keybinding, toggle option is necessary to not open multiple instances of the app.

If the app is invoked with the toggle option again, it should close.

Another option is to have the config flag e.g. keep only one instance That will behave in exactly the same way.

[undg]

current solution, as usual: https://github.com/undg/.dot/blob/master/bin/bin/pulse-remote-desktop-toggle