working support for loading keyboard mappings from .loopdrop folder

[ryanramage]

With this pull requests users can

• create a .loopdrop folder in their home dir
• add a json file in that dir for each hardware controller they have
• the json file shape looks like this https://gist.github.com/ryanramage/0f715d51424f1d10129cfa0c981f5bac
• the mixer and grid sections are optional depending if their hardware has keys, buttons, knobs, sliders, etc
• The user will be warned if a json file is invalid with an alert on loopdrop load
• If the json is valid, the user can add the controller by name in the UI to get a grid/mixer view

[mmckegg]

Hey @ryanramage,

Thanks so much for putting this work in! I'm very sorry that I haven't yet found the time and motivation get around to reviewing this. It is definitely something that I am interested in adding support for.

I guess my hesitation is that I'm keen to refactor the way loop-grid controllers are organised in the source code very soon. Right now there is so much duplication between all of the different controller types. Most of the code is just copied and pasted, and I'm starting to get pretty sick of managing it all whenever I make a change.

This PR adds even more duplication, so thing I'm gonna put this on hold until i do the refactor, and then I'll see how we go!

In a lot of ways, the idea behind your PR could become the basis of this new refactor. Maybe we can move all controller mappings into JSON. Will have to experiment to see if we can do all of the light stuff correctly across different controllers.