Fix update issues in BStringColumn.

In some cases, BStringColumn wouldn't properly detect that an update was
needed, and would consequently fail to truncate a string as needed with
a column resize.
Change truncation mode for image/function list views.

Since for image/file paths, the most interesting part is generally the
filename at the end, and the initial path information is mostly
redundant, it makes more sense to truncate at the beginning.