build: bump libmpg123 version

The minimum required version was bumped in the old configure script, but
for the waf build system is was somehow forgotten or overlooked.
Probably happened while the waf build system was developed in a separate
branch.

Closes #546.
player: select subtitles added with sub_add

In particular, this affects drag & drop of subtitles, which uses sub_add
internally. This will make the subtitles show up immediately, instead of
requiring manual selection of the added subtitle.

Might be not so ideal when adding multiple subtitles at once, because
that leads to multiple sub_add commands, and will end up with the last
subtitle instead of the first selected. But this is a minor detail.