Other corrections

This is a group of various tiny functions to normalize the subtitles.

The options are:

• Sort by start timestamp: The subtitles will be sorted by the start TS, either in increasing or decreasing order, according to the selection.
• Trim spaces in subtitles: Remove leading and trailing spaces in each line in the subtitle text.
• Remove empty lines in subtitles: Remove lines in subtitles that contain no text.
• Remove empty subtitles: Remove subtitles that contain no lines or that have all lines empty.

The correction is executed using the Fix subtitles button.