subtitles/dfxp: timecode: examine div tags for timecodes

[as]

The current dfxp parser does not examine the contents of <div tags for timing information. This causes it to generate an empty manifest for some valid dfxp files. It is valid to have timing information in the <div, and I can provide more details and examples on request, but here are two files:

FILE A: http://flv.io/ABC142908_id.dfxp.xml
FILE B: http://flv.io/ABC142908_en-US.dfxp.xml

FILE B currently generates an empty manifest. This change fixes this.

I have made changes to two functions that examine timing information:

• dfxp_get_duration, which traverses the entire XML to compute the ultimate duration of the subtitle's content.
• dfxp_parse_frames, which traverses the XML in a similar fashion. It now looks at <p tags and reverts to the last-visited <div if there exists no timing information in the <p tag.

Additionally, there is set of helper functions to remove duplicate code, as reconciling my changes to work inline resulted in cruft. The resulting changes allow FILE B to be parsed, as well as combinations of FILE A and FILE B.

[erankor]

I'm sorry, but this pull is currently in a state that is very far from what I'm willing to merge.
I'll list some of the issues I'm seeing, but in general - I think this code has to be thoroughly revised to match the coding standards of this module.

Style:

• missing static on internal functions
• missing spaces before/after operators
• * next to the var name instead of type name
• blocks without {}'s
• _t suffix for struct typedefs
• dfxp_parse_timestamp0 is returning the result in two different ways
• commit history is dirty with unrelated changes and their reverts

Efficiency:

• no need to store the duration dfxp_timestamp
• no need to look for duration if we already got end time (or the opposite, whichever we try to fetch first...)
• dfxp_get_duration is now calculating the start time even though it doesn't care about it

Bug:

• last_div is not reset when walking up in the tree - meaning - it may fall back to using a div that is not an ancestor of the current p element.

[as]

Thanks for the feedback, please take another look now, @erankor . I updated the pull request with respect to the feedback provided.

The style you provided is not consistent everywhere in this module even before this PR (e.x., * next to the var name instead of type name). I did not fix that, but the changes here in this PR should be.