Add reopenWidget method

[gdpelican]

This is a little bit of mad science that I needed for Babble; I figured I'd offer it up; you may or may not want to support it in core.

This allows us to add or override existing methods / properties on widgets, similar to Ember's reopenClass method. I found myself needing it to overwrite the url() method in notification-item, because I want to point chat notifications to the full page chat. I call it like this:

import { reopenWidget } from 'discourse/widgets/widget'

reopenWidget('notification-item', {
  url() {
    console.log("I am a custom function!")
  }
})

Supporting this._super() as well would be ideal here, but Ember's got a whole crazy scheme for supporting that, which I don't really want to reimplement (I was able to get the super method in other ways)

[mention-bot]

Thanks @gdpelican for your pull request 👍. By analyzing the blame information on this pull request, I identified @eviltrout and @tgxworld to be potential reviewers.

[discoursebot]

You've signed the CLA, gdpelican. Thank you! This pull request is ready for review.

[eviltrout]

This seems good to me, but I do think it should be a PluginAPI and exposed that way, rather than plugins just requiring the method and reopening themselves.

[gdpelican]

Okay, I've updated this to be called this way:

    withPluginApi('0.1', api => {
      api.reopenWidget('notification-item', {
        url() {
          console.log('some new implementation')
        }
      })
    })

[eviltrout]

You'll also need to bump the API version, otherwise people don't have a version to tie this new function to.

[gdpelican]

Oh, don't quite follow that yet. Is this the current version here?

function getPluginApi(version) {
  version = parseFloat(version);
  if (version <= 0.6) {

[eviltrout]

Yes! If you just update that to 0.7 then anyone who uses 0.7 will be guaranteed to have your new function.

[gdpelican]

Cool, I've popped that up to 0.7 now. Let me know if there's anything else? @eviltrout

[eviltrout]

Thanks, it looks great!