progressbar: Change the way the progress gadget gets size

progressbar: Change the way the progress gadget gets size

Size of the progress element now grows also when it's close to 0 size.

Previously the size was clamped to the minimum size, now it starts
growing from the minimum size.
So for a 100px trough with a 10px min size progress, the sizes of the
progress element change like this:
        old     new
0%      10      10
5%      10      14
10%     10      19
20%     20      28
50%     50      55
100%    100     100