Assorted cherry-picks from the master branch for Godot 3.1.2 [1st batch]

Created by: akien-mga

For review by @hpvb to see if any of those commits should not have been cherry-picked (e.g. breaks compat, or could introduce regressions).

As mentioned in https://github.com/godotengine/godot/pull/30291#issuecomment-508174485, one can easily get back to the actual PR by hovering the cherry-picked commit hash.

This is a first batch, I'll add more commits to it in coming days.

Note that a number of commits have already been cherry-picked in the 3.1 branch over the past few months for 3.1.2.