Add per-device keyboard layout support

[fmleo]

Hi! My current setup has 2 keyboards, an external one which I use at home and the laptop one which is used mostly everywhere else.
My suggestion comes from my previous experience with other wlroots compositors:

sway

input type:keyboard {
    xkb_layout br
}

input "6700:39936:SEMICO___USB_Gaming_Keyboard" {
   xkb_layout us
   xkb_options 'compose:ralt'
}

hyprland

device:at-translated-set-2-keyboard {
    kb_layout = br
}

device:6700:39936:SEMICO___USB_Gaming_Keyboard {
   kb_layout = us
   kb_options = 'compose:ralt'
}

As seen in the examples above, in both compositors you can set keyboard options based on the device names returned by swaymsg -t get_inputs and the hyprland equivalent.

#2419 could be relevant to this issue.

[zougloub]

The input config is documented to be in [input] (input/...) however get_input_device_section which can get per-device config, is not invoked in the wf::keyboard_t::keyboard_t ctor. Is it possible that calling it then making the option wrappers use section->get_option(...) instead of config.get_option("input/...")would solve the problem?

[soreau]

Preliminary patch for per-keyboard configuration. @zougloub has confirmed that it works.