kcms/desktoptheme: use sentence case for combobox

Reason for the change

The HIG recommend sentence case for combobox entries. The theme filtering combobox in the Plasma theme kcm currently uses title case for its entries.

This change converts the labels to sentence case, and adds translation context. The Appium test is updated to match.

Test plan

Correct capitalization was verified visually.

Screenshots or screen recordings

Before	After

Bugs fixed

N/A