Create a "Learn" action for convenience

[Benjamin Klum]

Should behave a bit like the built-in MIDI learn.

Suggestion:

1. Detect the last touched target
2. Find the most appropriate ReaLearn instance
   • If there is a ReaLearn instance which already has a mapping with that target, choose this one
   • If the target is a track target and this track has a ReaLearn instance, choose this one
   • If not, choose the first-added instance
3. Find the most appropriate mapping
   • If the ReaLearn instance already has a mapping with this target, choose it
   • If not, add one
4. Open the ReaLearn instance floating UI and open the mapping detail UI
5. Activate "Learn source"

[Benjamin Klum]

re #3 Arm ReaLearn track when using learn action (known issue: sets track volume as last touched parameter)

→ <<cset c41b8e9025d2>>

[Benjamin Klum]

re #4 Use UI thread fix again because the other one is not stable and probably never can be stable, re #3 Fix known issue picking up track volume as last touched parameter

→ <<cset 858235c59060>>

[Benjamin Klum]

re #3 Show message if action is triggered but no ReaLearn instance is available in the current project

→ <<cset e54210fe0690>>