The HUD widgets are custom and always dark themed. When using a light
system theme like Breeze, the icons were rendered with a dark color,
which was bad because it didn't contrast with the dark HUD. This was
not an issue in full screen mode, because we force the whole of
Gwenview to use a dark theme, and therefore the icons rendered a light
color.
So, when setting an icon using `HudButton::setIcon`, this patch now
re-loads the icon using the `KIconLoader` global instance, setting a
custom palette with a white foreground color, so the loader loads a
light icon. `KIconLoader`'s palette is then reset as to not interfere
with other icons.
BUG: 380257
FIXED-IN: 18.08.0