applets/digital clock: hide timezones identical to the local tomezone

It's possible and valid for the user to deliberately add a dedicated
timezone entry for their local timezone, in addition to the
pre-configured default "local timezone" entry. This can be useful when
traveling, because then when you switch your current local timezone to
one that matches your travel destintion, your typical local timezone
still appears in the list of timezones without you having to add it
again.

Currently, when you do this and you're *not* currently traveling, you
end up with duplicate entries in the popup and tooltip that show the
same city twice. To avoid this, now the plasmoid hides the duplicate
entry from the popup and the tooltip, and shows the user a hint in the
config dialog indicating that this will happen.

To make this work in the tooltip, I had to refactor the layout a bit to
use a Kirigami.FormLayout and away from its prior implementation, which
was rather hacky and breaks badly when conditionally adjusting the model
contents.