QtDialog: don't install desktop file bits on Apple

Apple uses the dmg file which doesn't have .desktop support.
Revert "CTestCustom: Suppress sphinx warning about missing favicon"

This reverts commit 7f70ad89ffcef299009131765b497bed8d69766a.
We can actually fix the warning instead of suppressing it.
Merge topic 'CTestCustom-suppress-sphinx-icon-warning' into next

ebdb05ed Revert "CTestCustom: Suppress sphinx warning about missing favicon"
QtDialog: install the 128x128 icon

Also only install the icons to the icons directory.
Merge topic 'unix-icon-install' into next

fb13502f QtDialog: install the 128x128 icon
fcb4541a QtDialog: don't install desktop file bits on Apple