`Granite.accel_to_string` doesn't handle Above_Tab

### What Happened?

We recently added "Switch Group" shortcuts in Gala which use `Above_Tab` key. But looks like `Granite.accel_to_string` doesn't handle it, therefore this shortcut cannot be added to Keyboard Settings for now.

### Steps to Reproduce

Try to use `Granite.accel_to_string` with `Above_Tab` key.

### Expected Behavior

It should format it as "~" I guess.

### OS Version

7.x (Horus)

### Software Version

Latest release (I have run all updates)

### Log Output

_No response_

### Hardware Info

_No response_