pavuapplication: allow one window per server

It was reported[1] that some users want to use multipe pavucontrol instances simultaneously to control multiple remote servers. Since pavucontrol 4.0 we have allowed only one window at a time, however. This patch relaxes the one window rule so that there can be one window per server (or per PULSE_SERVER value, to be precise).

This is implemented by appending the PULSE_SERVER value to the application ID if PULSE_SERVER is set (otherwise the normal ID is used). This works, because Gtk::Application uses the application ID to detect already running application instances. It's pretty ugly to mangle the application ID, because there's supposed to be only one ID per application, but I'm not aware of anything that would break or misbehave because of this.

[1] https://lists.freedesktop.org/archives/pulseaudio-discuss/2019-March/030929.html