[meta] Dropping 10.8 support

[josh64x2]

@barijaona @Eitot I thought it would be best for us to discuss in a stand-alone issue since we don't have much luck finding each other on IRC/Matrix/Slack.

As a trial I have created two new branches from master on my fork:

1. legacy3.1.x which I imagine will serve the same purpose as legacy3.0.x but for 10.8 users.
2. modernisation which I envisage will replace master.

I'll try and cherry-pick commits in a sensible order from autolayout into modernisation and submit a PR to master so it can all be reviewed.

Does this sound acceptable?

[josh64x2]

We can revisit when we look at storyboards.

[barijaona]

Oooups ! Something was broken at master-test3 (manual merges always require attention…), here is a master-test4