Loss of keyboard accessibility for Inspector on elements with handles

1) select a barline or hairpin
2) press Tab

Result: a handle is selected. Not necessarily a bad thing in itself, but for all other elements, Tab takes you to the Inspector, and is in fact the only way to transfer focus there. So now that selecting these elements displays their handles, there is no way to get to the Inspector by keyboard from there.

Solutions could involve changing the Tab semantics for elements with handles, making sure F8 transfers focus to the Inspector on show, or implementing a new shortcut to move between the scoreview, inspector, palettes, and other open windows, rather than requiring a long sequence of tabs.

The tab semantic could be that single tab moves you to the Inspector rather than select a handle. Seems the arrow keys also select a handle, although they also move a line, and I don't think we want to lose that.

Even if we made F8 take focus on show, I'm not crazy about needing to toggle F8 on and off to transfer focus to the Inspector and more than I am about doing so for the palettes. Which is why I'm happy there is a palette search command to transfer there directly.

Having a command (like F6, although its currently used for the Selection Filter) to move focus on one screen area to another makes sense to me long term. We have way too many controls between the inspector, palettes, and toolbars for mere tabbing to be a viable way of finding your way around.

I'd rather not rush into a decision on this, so I'm fine with leaving this regression for 3.4 but reconsidering soon after - hopefully along with Peter's PR for improvising palette navigation.