Add support for LCDs

[helgoboss]

Current state:

• It's possible to program LCDs in EEL language by using the "MIDI script" source.
  • Needs programming skills.
  • You can't access any other data than the current numerical target value ... no track name, no nothing.
• It's possible and convenient to control the 7-segment display on MC (Mackie Control) devices in order to display the current (numerical) target value as percentage by using the the "Mackie Control" controller preset.

Solution approach:

• Displaying target properties which are not the current numerical target value could be achieved by letting targets (but also ReaLearn globally) expose key-value pairs and making them accessible in the already existing "MIDI script" source.
• "MIDI LCD" source which supports various ways of programming LCDs (builtin) and accessing the key-value pairs
• "OSC" source should support string type in same way

In general, usage should be straightforward:

• Controller mapping should expose one line on the LCD via LCD source as a named virtual control element "ch1/lcd/line1", not yet mapping any info, just defining the LCD type etc.
• Mapping selected track's name to that line (or any other globally reachable info):
  • Source: Virtual control element "ch1/lcd/line1"
  • Target: "Global/Project: General info (feedback only, for displays)"
    • Display expression: "{selected_track_position}. {selected_track_name}"
• Mapping formatted target value to that line:
  • Target: e.g. "Send: Volume"
    • Display expression: "{value_in_db} dB"
• Mapping send name to that line:
  • Target: "Send: Volume"
    • Display expression: "{target_track_name}"

Related: #380
Losely related: #187