Add support for binding F13-F24 keys in the fcitx configuration utility.

[Kusoneko]

Hello,

As part of the USB Human Interface Device (HID) specification, a keyboard is allowed to have up to 24 F-keys (F1, F2, ... F12, F13, ... F24). https://www.usb.org/sites/default/files/hut1_22.pdf on page 85, you can see the F13-F24 keys and their specific keycodes for USB HID keyboards. Although they are rarely found on actual keyboards, I have on my own keyboard 5 extra keys which by default are bound to F13, F14, F15, F16 and F17 respectively. Unfortunately, when binding keys in the fcitx configuration menu, all five of them rebind keys to "Empty" despite them being recognized in the evtest utility as F13-F17. Would it be possible to add proper support for these F-keys? Specifically, I intend to bind them to the various layouts I have through the Input Method Selector addon.

Thank you for any help,
Kusoneko.