Clarify encoding of g_get_prgname()/g_set_prgname()
As spotted by @chpe in !2159 (comment 1179063),
g_get_prgname() (and, by extension,
g_get_application_name() which defaults to returning the prgname) could return a non-UTF-8 name if it’s initialised from
argv, which is in filename encoding.
In most situations, the filename encoding is UTF-8, but that’s not necessarily always true.
The documentation for the functions should clarify what encoding they use (likely UTF-8 everywhere) and conversion should be done on the calls to
g_set_application_name() when initialising from a source in filename encoding.