Currently the viewer uses semi-private `EventBus.prototype.{_on, _off}` methods, to try and ensure that all internal viewer state is updated *before* any "external" listeners are invoked.
For all use-cases outside of the viewer, e.g in the integration-tests, the `EventBus.prototype.{on, off}` methods are supposed to be used instead.
Unfortunately this isn't currently enforced in any way, except (hopefully) during review, and generally speaking it's not really possible to prevent the semi-private methods being used (e.g. by third-party users).
Hence this patch adds a new `INTERNAL_EVT` property which is *not* exposed anywhere (neither in the API nor globally), and whose value is generated at build-time, that the viewer uses to mark its `EventBus` listeners are internal.
This allows us to remove the semi-private `EventBus` methods, which helps to simplify that class a little bit.
Working on PR 20784, I couldn't help noticing that this code can be improved a little bit.
- Only initialize `PdfTextExtractor` in development mode and MOZCENTRAL builds, since it's unused elsewhere.
- Re-factor how `PdfTextExtractor` waits for the viewer to be available/ready, by using existing (internal) events.
This simplifies the `PdfTextExtractor` class and removes its `setViewer` method, which improves general consistency since normally the viewer-components don't use such a method in that way (here it was effectively used as a stand-in for a `setDocument` method).
- Finally, while slightly unrelated, rename the `#getAllTextInProgress` field in the `PDFViewer` class to `#copyAllInProgress` to clearly indicate what it's for since the `getAllText` method is used more generally now.
