Fix UI menu auto-hotkey assignment

Fixes #7591

The lag comes from the inefficient loop that automatically assigns hotkeys.

The missing hotkey Z comes from a wrong magic value (`... < 61` should have been `... < 62` as 26+26+10==62).

I rewrote the loop and used a static string. One can easily add more possible hotkeys there, for example and as reference:

```
const std::string display_help_hotkeys=
"abcdefghijkpqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789:;'\",./<>?!@#$%^&*()_[]\\{}|`~";

const std::string inv_chars =
"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!\"#&()*+./:;=@[\\]^_{|}";
```

Let's see how often I have to fix this PR.