allow chapter duration of 0

[robUx4]

There can be segments with just chapters containing commands.

In ordered chapters the content will not play anything, it may seek to the
position for nothing but that's not a big deal.

Fixes #542

[robUx4]

When exporting a DVD to Matroska the "first play PGC" often just contains a command. In Matroska chapter that corresponds to an empty chapter with an enter (or leave) command.

Here's an example output of DvdMenuXtractor:

		<!-- First Play PGC -->
		<ChapterAtom>
			<ChapterUID>1729839304</ChapterUID>
			<ChapterTimeStart>00:00:00.000000000</ChapterTimeStart>
			<ChapterTimeEnd>00:00:00.000000000</ChapterTimeEnd>

	<!-- Enter your text here -->
	<ChapterDisplay>
			<ChapterString>First Play PGC 1729839304</ChapterString>
	</ChapterDisplay>

			<ChapterProcess>
				<ChapterProcessCodecID>1</ChapterProcessCodecID>
				<ChapterProcessPrivate format="hex">30 00 00 00 </ChapterProcessPrivate>
				<!-- Pre commands -->
				<ChapterProcessCommand>
					<ChapterProcessTime>1</ChapterProcessTime>
					<ChapterProcessData format="hex">02 53 00 00 00 00 0f 00 00 30 06 00 00 00 42 00 00 </ChapterProcessData>
				</ChapterProcessCommand>
			</ChapterProcess>
		</ChapterAtom>

So it is needed in ordered chapters. It doesn't make a lot of sense in unordered chapters, but could be used as a marker of position (for example in an audio or video editor).

We do say that the end value is timestamp excluded. So we may clarify what to do with this when the start is equal to the end as it means the same timestamp is both included and excluded.

[robUx4]

Fixed some typo for greater than or equal to .