Alt+left/right should remember last selection

https://github.com/musescore/MuseScore/pull/5221

I appreciate the questions and opportunity to discuss!

With my change, pressing Esc does still lose the selection. The only difference is that either Alt+Left or Alt+Right will get it back. So it's still the case that after pressing Esc, press Delete, for instance, does nothing And it's still the case that you can get back to the beginning of the score by pressing Ctrl+Home. Someone not using Alt+Left/Right won't see any difference in behavior; no other commands attempt to restore the selection.

The reason for the change it is that it is easy to accidentally press Esc and lose your selection, which for a sighted person is only a minor annoyance, but for a blind person is much more frustrating as it becomes very difficult to get back to where you were in navigation. This is something that constantly comes up in accessibility reviews, and we've fixed quite a few bugs over the years with selection getting lost specifically because of accessibility concerns.

While it would theoretically be possible to prevent Esc from losing the selection if Qt reports that an accessibiity device such as a screenreader is being used, that's not really idea, as not all people who might benefit from this would be using such devices (eg, people who can see but have mobility issues preventing use of a mouse). And while I've fixed the more obvious cases where selection was lost accidentally, there are almost certainly others, so we really need a strategy for recovering selection.. This fix improves life for blind users significantly while not affecting the behavior of any commands except Alt+Left/Right.

Thanks for the feedback! My suspicion is most sighted users have no idea that Alt+left/right exist, so most people won't notice a change, but for those few who do rely on Alt+left/right to gain more keyboard control and to reduce reliance on the mouse, the change will simply be welcome. For blind users, though, it's pretty much essential.

With my implementation, pressing Esc does always remembers the last selected element. And it would also be remembered any other time the selection is accidentally lost (although of course if there are such bugs remaining, I'd like to fix them too).

But currently, in my PR, the only way to regain the remembered selection is to use Alt+left/right. The behavior using plain left/right is not changed. I did that to avoid complaints that we changed behavior for no reason. Also, for example, I could imagine us someday implementing left/right in normal mode with no selection as a way of moving the canvas.

But anyhow, it's simple enough to add similar logic to ordinary left/right commands. or add a new command whose function is to restore the last selection (and perhaps do a full read on the localtion - see #293301: Add accessibility command to force full read of current location). But there is no hurry on any of that to me. What's near-critical is making sure Alt+left/right always do something reasonable, and my PR accomplishes that.