kcms/soundtheme: Fix items overflowing the card

Allow the label to shrink and elide, so it doesn't push the
buttons out of the card's frame, which can happen with larger
fonts or in more verbose languages.

With this also drop the "compact mode" concept and just place
the general preview button above. It simplifies the layout and
avoids UI jumpiness and edge cases.

We cannot currently set a minimum size for the cards right now,
so let's adapt to it the best way possible.

BUG: 480554
FIXED-IN: 6.0

(cherry picked from commit 3e1bb77e)