Remove support for the unmaintained GTK UI plugin

[gber]

This removes support for the GTK UI plugin but leaves support for the GTK control center in, as requested in #301.

[gber]

Can someone please review this, I think I addressed the issue raised in #301?

[jreidinger]

@jsrain can you please check it?

[mvidner]

A reference to Bugzilla or Fate is missing. I think bnc#901851 should be mentioned in .changes.

[gber]

I think bnc#901511 is more suitable due to the discussion there and it being referenced in the release notes as well. I've updated it accordingly.

[mvidner]

LGTM, merging.
Thank you, Guido!

[mvidner]

@gber concerning applying this as a maintenance update for 13.2, it is not as simple as only using this code. People would suddenly get the ncurses UI instead of the GTK UI even in cases where they had no problems with GTK.

[gber]

In 13.2 the x11_yast pattern recommends libyiu-pkg-qt6 which requires libyiu-qt6, the x11_yast pattern also supplements the x11 and yast2_basis patterns. All desktop patterns either depend on the latter two patterns or directly on x11_yast, also note that both the KDE and GNOME patterns already default to Qt anyway, so AFAICS this should not be a problem in practice?

[gber]

@mvidner So, given the above, can you please elaborate? Otherwise I'd really like to push an update to 13.2 rather than leaving people with a broken YaST frontend for which any bug reports will be ignored (and given the stuff that is on b.o.o I think it is impossible not to have problems with GTK if you use the GUI).