Prefer m3u8 over m3u as a playlist format
m3u files, when loaded by an audio player, will cause it to choke on entries with non-ASCII characters--even if the file itself is UTF-8 encoded. To make this work, the filename extension itself must be m3u8, not m3u.
Elisa already transparently supports files with the .m3u8 extension, but does not save in this file format by default, and does not list them for opening in its open dialog.
This commit makes Elisa able to open m3u8 files from its file dialog and save playlists in that format by default.
This MR does not fully fix https://bugs.kde.org/show_bug.cgi?id=389187. Renaming a playlist file that contains non-ASCII to change its filename extension from .m3u to .m3u8 works, but I can't find a way to force QMediaPlaylist to successfully interpret playlists with the .m3u filename extension as if they were m3u8 files. In theory passing "m3u8" to the format argument of QMediaPlaylist::load() should do this, but it does not seem to work.