fixed an issue with tab reverse on windows.

[MCord]

This PR fixes a small but annoying bug on windows that prevents SHIFT + TAB to select the tabs in revers order. The bug is caused by the difference in interpretation of modifiers either in crossterm or lower levels.

for non windows platforms the SHIFT + TAB key generates KeyCode::BackTab without any modifiers while on windows the same produces KeyCode::BackTab + Shift modifier.

I am not sure the issue should be addressed in gitui but here is the fix.

[extrawurst]

Oh damn, we had such an issue before: #26
Can you do me a favour and tackle this upstream like I did in the previous one, the guys are more than happy to accept such fixes in crossterm.

[MCord]

Sure, I will look into it.

[extrawurst]

@MCord cool I created a followup issue to keep track of this: #112