Feature: optional JackTransport support

[cbix]

• Configurable display of transport clock / bpm
• transport buttons in menu
• forward/rewind when scrolling on panel widget (opt-out)
• remember widget can have arbitrarily small space, e.g. in material-shell

Until we have an official jackdbus interface for transport control:

• implement a simple libjack-based service that provides a dbus interface
• should be consistent with the rest of the jack API and dbus interfaces
• if the service is not installed, hide the feature completely
• never register as master

https://github.com/jackaudio/jack2/blob/develop/common/jack/transport.h