Fix arrow keys not working in tag editor on qt 6.8

[iamllama]

Fixes #3751

On windows and on qt 6.8, the arrow keys' keydown events now have codes of the form NumpadX instead of the expected Arrow{Left,Right,...}. Delete's also became NumpadDecimal. Not sure if this is unique to my system, but according to mdn, it's known to be problematic:

For example, the code returned is "KeyQ" for the Q key on a QWERTY layout keyboard, but the same code value also represents the ' key on Dvorak keyboards and the A key on AZERTY keyboards. That makes it impossible to use the value of code to determine what the name of the key is to users if they're not using an anticipated keyboard layout.

To determine what character corresponds with the key event, use the KeyboardEvent.key property instead.

I've also added bottom and right margins to the tag editor, as the focus outline was getting cut off there (split off into #4115)