Improve the subtitle editor

This issue is related to MR !522 (merged).

With the improved support for ASS format subtitles in the ongoing work, we can now store and apply more complex subtitle styles. The subtitle editor need to be improved to better deal with subtitle styles. This issue is for collecting and discussing ideas.

• Set styles for each subtitle
• Help users input ASS override tags
• Set the global subtitle style in another place (maybe the "Manage Subtitle Styles" widget)