Prefer KIconLoader::Small icon sizes for window thumbnails when possible

This makes the pager actually abide by the system settings designated
Small icon size. So increasing it to e.g. 42x42 would make the window
icons in the pager use that size.

It's set to use the width/height (whichever is smallest), and the
KIconLoader::Small. Whichever value is smaller.

Was previously hardcoded to 16x16. Should be a safe change, going to
backport.
BUG:25766
(cherry picked from commit 30366dfdcb6e649aca7edcf8fb9ea1b4a8fa3aa7)