Add JACK meta data support

[SpotlightKid]

This is an umbrella ticket to gather ideas on where JACK meta data could be beneficial. Please add implementation tickets / PRs for individual parts of the application where meta data support is or should be added, so progress can be properly tracked.

• Set JACK_METADATA_ORDER on all input and output ports. maintaining the order in which channels are shown in the UI.

• Set JACK_METADATA_PRETTY_NAME for all input and output ports with the channel label. Convert channel label to identifier-format (i.e. no whitespace or non-ascii chars) to use as the port name/symbol.

• Allow to set the JACK_METADATA_PRETTY_NAME of jack_mixer's JACK client, so different instances of jack_mixer can be identified better in a JACK patchbay (see also Allow to set the client name from UI #45). Might be a new menu entry in teh "Mixer" menu. Should be saved into the session file, not the global settings.

• Set JACK_METADATA_PORT_GROUP on all ports to group ports belonging to a channel. Also add port groups for monitor outputs and channel direct outputs.

• Set JACK_METADATA_ICON_NAME for jack_mixer's JACK client.

Since setting meta data requires changes to the C-extension, we should probably hold off implementing any of this until #29 is ready for merging.