We have a fair number of (effectively) single-line event handlers in the `web/app.js` file, which leads to unnecessarily verbose code. These can, without affecting readability too much, be replaced either by:
- Using `bind` for the simplest cases.
- Using arrow-functions for the remaining ones.
Note that this is possible since we started removing event listeners with `AbortSignal`, which means that we no longer need to keep a reference to the event handler functions to be able to remove them.
Given that the old event handler functions use fairly long function names, and the way that they access `PDFViewerApplication` (given their scope), they impact the overall code-size unnecessarily.
*Note:* This patch reduces the size of the `gulp mozcentral` output by `~3.7` kilo-bytes, which isn't a lot but still cannot hurt.
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