Write dive data as video subtitles

This commit adds an entry to the dive media context
menu which offers to write a subtitle file. This
creates an .ass file for the selected videos.

In an attempt to to clutter the screen too much, don't
show irrelevant entries (zero temperature or
NDL and show TTS only for dives with stops).

VLC is able to show these subtitles directly, they
can be integrated into the video file with ffmpeg.

Signed-off-by: Robert C. Helling <helling@atdotde.de>

core/save-profiledata.h

@@ -9,6 +9,7 @@ extern "C" {
 #endif
 
 int save_profiledata(const char *filename, bool selected_only);
+void save_subtitles_buffer(struct membuffer *b, struct dive *dive, int offset, int length);
 
 #ifdef __cplusplus
 }