support tempo-sync with JACK

[wolftune]

Plugins that don't even necessarily do this themselves should still be made to work in Carla.

Carla could simply have a way to make a parameter be synced in some relative way to the JACK tempo. This is especially useful for things like echo / delay.

[falkTX]

This is now fixed, but it only works for LV2 plugins.

Later I want this available like the midi-cc options, so any parameter can sync to any kind of transport info (bpm, frame, etc).
later...

[mxmilkiib]

To get the tempo number, https://github.com/ryukau/LV2Plugins/tree/master/lv2cvport/CV_TimeInfo

Then one can use a math CV plugin to half, third etc that figure to do things with.