dash: Revert mislead cleanup

When adjusting dash icon sizes, we compute the icon padding by subtracting
the configured icon size from the first icon actor's preferred size. To
make sure that the preferred size correctly corresponds to the current
dash icon size even while the icon is animating, we enforce the size
before the size request. For that we used to temporarily manipulate
the icon texture size directly, but commit e92d204d cleaned this
up to use the setIconSize() method instead.
This does not work however, as the icon actor's iconSize property will
always match the dash iconSize property, making the method a noop. So
go back to the original approach of enforcing the texture size to make
sure we always base our computations on correct values.

https://bugzilla.gnome.org/show_bug.cgi?id=745649