In recent versions the table headers were no longer bottom-aligned
(if more than one line). This patch fixes that by using minipages
for table headers in non-simple tables.
Closes#7347.
In 2.11.3 we started adding `\addlinespace`, which produced less
dense tables. This wasn't an intentional change; I misunderstood
a comment in the discussion leading up to the change. This commit
restores the earlier default table appearance.
Note that if you want a less dense table, you can use something like
`\def\arraystretch{1.5}` in your header.
Closes#6996.
Note that the multirow package is needed for rowspans.
It is included in the latex template under a variable,
so that it won't be used unless needed for a table.
The 3 native table test cases are normalized so that it will looks exactly like it is written by some pandoc writers.
Note that apart from white space normalization, it includes other normalization such as `[Str "Nordic countries"] to [Str "Nordic",Space,Str "countries"]`.
