Use new Jack Metadata API to mark CV ports.

By adding metadata key "http://jackaudio.org/metadata/signal-type" value "CV" pair to CV ports, patch bays can discriminate between audio and CV ports (SVN patchage e.g. can already do so).

Metadata API is only implemented in Jack1 for now, this patch will thus have no effect when linked to Jack1/2 without support for metadata.
Maybe silence -Wunused-result warnings on arch.
Timeline: Move punch logic to record diskthread. Punches are now sample accurate and work better in freewheeling mode.
Timeline: Avoid deadlock in diskthread.
Timeline: Fix bug where opening context menu on a region is equivalent to initiating a drag on that region.

Closes #126
Timeline: Add more more keybindings, particularly of performing actions at the location of the playhead.
Mixer: Don't segfault when user right-clicks on channel number module.

Closes: #128
Timeline: Don't die when undo function is used when no project is open or when there is nothing left to undo.
NSM: Fix typo in --help text.

Closes #137
NSM Proxy: Detect, log, and display startup errors of proxied process.