kcms/desktoppaths: always perfectly fit text

Previously the text fields' widths were hardcoded to gridUnit * 10. This
was not long enough for some text. Now, the fields all share a common
width that's equal to width long enough to fit the longest piece of
text, limited to a sane value to avoid overflowing the window.

BUG: 471151
FIXED-IN: 6.0