*Please note:* Given the pre-existing issues raised in PR 13056, which seem to block immediate progress there, this patch extracts some *overall* improvements of the scripting/sandbox destruction in `PDFScriptingManager`.
As can be seen in `BaseViewer.setDocument`, it's currently necessary to *manually* delay the `PDFScriptingManager`-destruction in order for things to work correctly. This is, in hindsight, obviously an *extremely poor* design choice on my part; sorry about the churn here!
In order to improve things overall, the `PDFScriptingManager._destroyScripting`-method is re-factored to wait for the relevant events to be dispatched *before* sandbox-destruction occurs.
To avoid the scripting/sandbox-destruction hanging indefinitely, we utilize a timeout to force-destroy the sandbox after a short time (currently set to 1 second).
By moving this code from the `BaseViewer` and into `PDFScriptingManager`, all of the scripting initialization/handling code is now limited to just one file/class which help overall readability (in my opinion). Also, this patch is a *net reduction* in number of lines of code which can never hurt.
As part of these changes, the intermediary "pageopen"/"pageclose" events are now removed in favor of using the "regular" viewer events directly in `PDFScriptingManager`. Hence this removes some (strictly unnecessary) indirection in the current code, when handling PageOpen/PageClose events, which leads to overall fewer function calls in this part of the code.
The *main* purpose of this patch is to allow scripting to be used together with the viewer components, note the updated "simpleviewer"/"singlepageviewer" examples, in addition to the full default viewer.
Given how the scripting functionality is currently implemented in the default viewer, trying to re-use this with the standalone viewer components would be *very* hard and ideally you'd want it to work out-of-the-box.
For an initial implementation, in the default viewer, of the scripting functionality it probably made sense to simply dump all of the code in the `app.js` file, however that cannot be used with the viewer components.
To address this, the functionality is moved into a new `PDFScriptingManager` class which can thus be handled in the same way as all other viewer components (and e.g. be passed to the `BaseViewer`-implementations).
Obviously the scripting functionality needs quite a lot of data, during its initialization, and for the default viewer we want to maintain the current way of doing the lookups since that helps avoid a number of redundant API-calls.
To that end, the `PDFScriptingManager` implementation accepts (optional) factories/functions such that we can maintain the current behaviour for the default viewer. For the viewer components specifically, fallback code-paths are provided to ensure that scripting will "just work"[1].
Besides moving the viewer handling of the scripting code to its own file/class, this patch also takes the opportunity to re-factor the functionality into a number of helper methods to improve overall readability[2].
Note that it's definitely possible that the `PDFScriptingManager` class could be improved even further (e.g. for general re-use), since it's still heavily tailored to the default viewer use-case, however I believe that this patch is still a good step forward overall.
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[1] Obviously *all* the relevant document properties might not be available in the viewer components use-case (e.g. the various URLs), but most things should work just fine.
[2] The old `PDFViewerApplication._initializeJavaScript` method, where everything was simply inlined, have over time (in my opinion) become quite large and somewhat difficult to *easily* reason about.
