fix placement of placeholder text in QLineEdits with action icons

fix placement of placeholder text in QLineEdits with action icons

Origin: upstream, https://code.qt.io/cgit/qt/qtbase.git/commit/?id=0e6b31019f01c72e
Last-Update: 2021-08-10

After dc794f7622bc00f7ca50fab65d6965695d6d2972, side widgets only got
space if they were not fading out, but the logic was not correctly
accounting for side widgets that never fade, such as buttons added via
QLineEdit::addAction.

Fix this to give visible widgets space, unless they are fading out. That
was the intent of the original change. Rename the variable to make its
purpose clearer, and reset it at the end of the fade-out animation.

Add a much-needed test that relies on private APIs to verify that the
effective margins are calculated correctly.

Gbp-Pq: Name fix-placement-of-placeholder-text-in-QLineEdits-with-action-icons.diff