Make the Shift-Apple-` and Shift-Apple-Tab keypresses cycle backwards through open windows

[c0mmander8]

As referenced in issue #154, make the Shift-Apple-` and Shift-Apple-Tab keypresses cycle backwards through open windows.

[inexorabletash]

Shift can't be detected on all systems, so Shift+Apple+Tab may not be worth it. Apple+~ should be pretty easy, though.

[inexorabletash]

On the IIgs, KEYMODREG Bit 0 should provide Shift key detection.

On the Platinum IIe, BUTN2 Bit 7 would provide Shift key detection per (https://retrocomputing.stackexchange.com/questions/11395/what-combinations-of-modifier-keys-can-you-detect-on-an-apple-ii), but determining that model can't be done programmatically. We could grab the button state on startup (on local testing with a non-Platinum IIe, I always see it as high) as a mask.

But since this would be very inconsistent to document, I'm just going to leave it to just Shift+Apple+` since that is unambiguous.

[c0mmander8]

Is your feature request related to a problem? Please describe.
I'm always frustrated when I am using Apple II DeskTop on an Apple IIGS or on a Platinum Apple IIe and familiar keyboard shortcuts from other platforms involving the Shift key do not work.
Describe the solution you'd like
Please reopen this issue and, as referenced above and in issue #373, as an extra bonus for those users of Apple II DeskTop using an Apple IIGS, a Platinum Apple IIe or any other model that can detect the Shift key being pressed, make the Shift-Apple-Tab keypress cycle backwards through open windows in addition to the already existing Shift-Apple-` (Apple-~) keypress.

Describe alternatives you've considered
Keep on using the Shift-Apple-` (Apple-~) keypress to cycle backwards through open windows, just as I have always done.

Additional context
Despite the added inconsistent documentation, this issue is extremely handy for when I am using Apple II DeskTop on an Apple IIGS or on a Platinum Apple IIe and will be able to use the Shift-Apple-Tab keypress to cycle backwards through open windows without thinking about which keypress combination I have to use, just like I do on other platforms.

[c0mmander8]

I see that the make the Shift-Apple-` keypress cycle backwards through open windows part of this issue was fixed in commit 1c4acb2.