Some builds of Qt appear to have a bug in icon loading: QIcon::hasThemeIcon
is sometimes unaware that the icon returned from QIcon::fromTheme was a
fallback icon, causing hasThemeIcon to incorrectly return true.
In qTox this leads to the tray and window icons using the static qTox logo
icon instead of an icon based on the current presence status.
This workaround checks for an icon that definitely does not exist to
determine if hasThemeIcon can be trusted.
On systems with the Qt bug, this workaround will always use our included
icons but user themes will be unable to override them.
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