Move bstr_strip_ext() and bstr_get_ext() to path_utils.c making them use
split_ext(), which is better than the previous bstr.c implementation
because it doesn't consider . in the dirname or at the start of
basenames.
There are at least two users of this functionality, the filename/no-ext
property and the screenshot template %F specifier. The latter uses a
buggy private version. So split out what the former does, so it can be
reused.
Co-authored-by: Guido Cella <guido@guidocella.xyz>
Guido Cella