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

I took the screenshots in Spanish, just later realized it is quite worse in French

	BEFORE	AFTER
System Settings minimum width
Worst case