Last update we had about this was here: construct.net/en/forum/construct-3/general-discussion-7/c3-plan-possible-create-143748
Last ashley response I've seen about the subject was
I'm afraid I have no update to provide. We deal with hundreds of feature requests, thousands of bug reports, our own internal priorities, and all the other support and administration that comes with running a software company, and we have just three developers. We simply don't have the resources to tackle everything quickly, let alone huge projects such as this one that would likely involve overhauling how much of Construct itself works.
So, unfortunately looks like it'll take a while for plugin making in editor become a reallity. :\