Refactor the `selectScaleOption` function, in `Toolbar._updateUIState`, to prevent any possible future display glitches
Refactor the `selectScaleOption` function, in `Toolbar._updateUIState`, to prevent any possible future display glitches

Since the localization service is now asynchronous, depending on the load the browser is under, there's a small risk that the lookup of the 'page_scale_percent' string could be delayed slightly.
If the scale would change a couple of times in *very* quick succession, there's perhaps a *theoretical* possibility that the Zoom dropdown would display an incorrect value.

Consider the following, somewhat contrived, theoretical example of two zoom commands being executed *right* after one another:
```javascript
PDFViewerApplication.pdfViewer.currentScale = 1.23;
PDFViewerApplication.pdfViewer.currentScaleValue = 'page-width';
```

Only the `currentScale` call will currently trigger a l10n lookup in `selectScaleOption`. However, as far as I understand, there's no *guarantee* that the l10n string is resolved *before* `selectScaleOption` is called again as a result of the `currentScaleValue` call.

This thus has the possibility of putting the Zoom dropdown into an inconsistent state, since it's currently updated synchronously for one code-path and asynchronously for another.

To avoid these issues, I'm proposing that we *always* update the Zoom dropdown asynchronously, such that we can guarantee that the ordering is correct.

mozilla/pdf.js,Shoobx/pdf.js,skalnik/pdf.js,mozilla/pdf.js,nawawi/pdf.js,websirnik/pdf.js,xavier114fch/pdf.js,skalnik/pdf.js,mainegreen/pdf.js,timvandermeij/pdf.js,caremerge/pdf.js,Rob--W/pdf.js,Shoobx/pdf.js,nawawi/pdf.js,caremerge/pdf.js,Snuffleupagus/pdf.js,Snuffleupagus/pdf.js,xavier114fch/pdf.js,timvandermeij/pdf.js,websirnik/pdf.js,timvandermeij/pdf.js,mainegreen/pdf.js,nawawi/pdf.js,adlerweb/pdf.js-Touch,Rob--W/pdf.js,showpad/mozilla-pdf.js,macroplant/pdf.js,Snuffleupagus/pdf.js,Shoobx/pdf.js,macroplant/pdf.js,macroplant/pdf.js,showpad/mozilla-pdf.js,mukulmishra18/pdf.js,mukulmishra18/pdf.js,showpad/mozilla-pdf.js,mozilla/pdf.js,adlerweb/pdf.js-Touch