dash: Allow icon sizes equal to the available space

When checking for a suitable icon size, Dash currently checks which of the hardcoded icon sizes is smaller than the calculated available size.

On some circumstances, however, when the calculated available size is exactly equal to the hardcoded icon sizes, Dash selects a smaller size. This cascades (the next icon size is exactly the smaller size, etc) and ends up with always Dash selecting smallest size available, even with plenty of available space.

Check if the calculated available size is smaller or equal to the hardcoded icon sizes.