[Feature Request] Using dropdown menu for Indexes

[glu8716]

At the moment, to find out which indexes are not used in the base, you have to scroll through them one by one using the little arrow (up or down), until you see the red square appear in the top-left corner and the Decoration is set to "None". I believe having a dropdown menu listing all indexes, perhaps with a checkmark next to each to indicate whether it's already in use or not, could be more intuitive.
Something like:

[✓] Index 1
[✓] Index 2
[✓] Index 3
Index 5
[✓] Index 6
[✓] Index 7
[✓] Index 8
Index 9
[✓] Index 10
[✓] Index 11
[✓] Index 12
[✓] Index 13
[✓] Index 14
Index 15

The user can then select them in the dropdown menu.

[ScottehMax]

I'd like to improve the usability of the actual base editing part at some point, but given PKHeX already has some support for secret base editing, I'm wary of doing work which will be made obsolete when they add a layout editor.