v0.5.0 issues for hotfix

[low-sky]

There are a two issues with v0.5.0 as seen on Mac. Some appear to be with the histogram.

• Images in the SmallMultiples test suite produce a crash when opened: cubelog.txt This issue goes away when the histogram plugin is closed. If open the plugin again and then make the link, it crashed immediately upon linking.
• If I create an image stack by loading three images from the SmallMultiples test data with the histogram viewer closed so it doesn't crash, I can trigger something that runs the processor up to 100% and hangs. To reproduce
  1. Load three images from the SmallMultiples directory
  2. Open the Image Settings > Stack Menu,
  3. Unselect Auto Select
  4. Click one image, Select Plus compositing.
  5. Click another image ... hang.
     The current code may fix these.

[slovelan]

I have not been able to reproduce the SmallMultiples test suite crash with the current code base. I did not test all of the SmallMultples images, but I did test the first 3 with the histogram plugin open and did not experience a crash. Unless there is something I am missing when I try to reproduce it, it appears fixed.

[slovelan]

I have also not been able to reproduce the second issue, the hang, with my local code base.

[slovelan]

By accident, I stumbled on a reproduce of the second issue, and a fix should be in with my most recent commit.