Fix enum end

[alxvth]

Fixes a wrong UI heading:

#114 added a couple of entries to the ImageCollection::Column but did not adjust end End entry. That is referenced in ImageCollection::Image::Column for setting the first value. This let to the situation where this switch could never be reached:

ImageLoaderPlugin/src/ImageCollectionsModel.cpp

Line 548 in 3962865

switch (static_cast<ImageCollection::Image::Column>(section)) {

Instead of using "Dimension name" a column heading in the loader dialog would show "Mirror vertically".