Switch keyboard layout viewer to tecla

[Jeremy Bícha]

Impact
======
GNOME 45 switched their default keyboard layout viewer from libgnomekbd to tecla. This change was made in gnome-control-center but Debian and Ubuntu carried a patch to revert this for gnome-shell because gnome-shell wasn't calling tecla correctly which made the keyboard layout viewer not work correctly for some keyboard layouts.

The gnome-shell bug was fixed in 46.2.

Tecla has these advantages compared to libgnomekbd:
- Uses GTK4 and libadwaita
- Which means improved support for the dark style preference (Ubuntu enables dark style in many more places than GNOME normally does so the libgnomekbd viewer has partial dark style support in Ubuntu.)
- Much more modern looking window
- Consistency with the existing ⋮ > View Keyboard Layout in GNOME Settings > Keyboard
- Does what every other GNOME 45 or GNOME 46 distro does
(Screenshots attached to show differences)

Therefore, this SRU drops the revert patch, drops Recommends: gkbd-capplet and adds Depends: tecla. apt should mark gkbd-capplet for autoremoval since nothing in the default Ubuntu Desktop Depends or Recommends it any more. It is harmless if either gkbd-capplet is uninstalled or kept installed.

gnome-control-center already has Depends: tecla so this is not installing anything new.

Test Case
=========
From a terminal, run sudo apt install language-pack-gnome-fr
Install the gnome-shell update. Log out and log back in.
Open the GNOME Settings app (gnome-control-center).
In the left sidebar, click Keyboard.
Click Add Input Source…
In the dialog, choose French (France), then choose French (AZERTY) then Add.
In the right side of GNOME Shell's top bar, the keyboard layout switcher should be visible with en. Click en then change the keyboard layout to French (AZERTY).
Click Show Keyboard Layout.
A popup should show a keyboard layout with the second row of characters beginning with a z e r t y as shown in the tecla-azerty-light.png or tecla-azerty-dark.png screenshots.
Close the keyboard layout popup.
Switch the keyboard layout back to en
Click Show Keyboard Layout
The popup should display with the keyboard layout set to your usual English keyboard layout.

What Could Go Wrong
===================
This is a fairly minor feature and should not be essential to anyone who needs alternate keyboard layouts. I would guess people would be more likely to click the ⋮ > View Keyboard Layout option in GNOME Settings > Keyboard when choosing input sources rather than via GNOME Shell. And GNOME Settings was already set to use Tecla.

This change does not affect actual usability of alternate keyboard layouts, just the single button to view the layout.

Other Info
==========
This change means that libgnomekbd will be demoted from main to universe for Ubuntu 24.10. For technical reasons, that demotion is not possible for already released stable versions of Ubuntu like Ubuntu 24.04 LTS.