Merge ~ogayot/ubiquity:fix-oem-config-showing-navigation into ubiquity:main

Hide navigation buttons on last KDE screen with oem-config

When oem-config is run as part of an OEM install in a kubuntu
environment, the last screen (which shows progression) has clickable
Next and Back buttons (aka., the navigation buttons). Clicking on the
Next button makes the application crash with:

Traceback (most recent call last):
 File" /usr/lib/ubiquity/ubiquity/frontend/kde_ui.py", line 1073, in on_next_clicked
  ui = self.pages[self.pagesindex].ui
IndexError:list index out of range

When doing a normal install, the Next & Back buttons are hidden on the
last screen ; which is the right thing to do.

The function that hides the navigation buttons is start_slideshow but it
only hides the navigation if a slideshow exists and is not intentionally
disabled.

When running oem-config as part of an OEM install, the slideshow does
not exist so the navigation buttons end up still being visible.

When doing an OEM install on kubuntu (KDE), the final screen of oem-config has clickable Back and Next buttons.

Clicking on the Next button crashes the install.

When doing a normal install, the Next & Back buttons are hidden on the
last screen ; which is the right thing to do.

The function that hides the navigation buttons is start_slideshow but it
only hides the navigation if a slideshow exists and is not intentionally
disabled.

In our scenario, the slideshow does not exist so we end up leaving the navigation buttons visible and clickable.

Another way to fix the issue would be to make start_slideshow hide the navigation no matter what, but I don't have enough experience with ubiquity to check if it could break something else.

This is the original bug report:
https://bugs.launchpad.net/ubuntu/+source/ubiquity/+bug/1990196

Ideally, this patch should be SRUed to series for which we intend to spin new installer images (i.e., jammy and kinetic).