Our back shortcut is registered as Alt+Shift+Backtab but our converted
Qt key coming into the test method is Alt+Shift+Tab. The logic so far
made this always fire for the normal Alt+Tab shortcut as at some point
the Shift modifier gets removed to do the test.
To handle it properly we first have to extract all the modifiers to just
get the key. If the key is Tab, we replace it with Backtab, combine it
with the extracted mods so it will be Alt+Shift+Backtab which matches
the registered shortcut.
The existing backtab solution can probably be removed and is clearly
wrong as it uses the keys as flags which they aren't.
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