Use `pagehide` event instead of `unload` in the following cases:
* For classic editor to release the post lock.
* In Text widget to rebuild editor after dragging widget to new location in classic widgets interface.
* To clear out the `window.name` when navigating away from a post preview.
* To suspend heartbeat, while also using `pageshow` event to resume as if it had been a focused tab in case page restored from bfcache.
Also:
* Remove obsolete mobile cleanup code in `js/_enqueues/lib/gallery.js` (introduced in [9894]). Do same for `src/js/_enqueues/wp/media/models.js` (introduced in [22872]). See #22552.
* Remove obsolete Firefox-specific workaround in `js/_enqueues/wp/mce-view.js` from [39282]. See #38511.
Fixes#55491.
Props spenserhale, westonruter, adamsilverstein, azaozz, shawfactor, peterwilsoncc, swissspidy.
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