This brings the MP4 API in line closer to other tag formats and makes it
possible to access the tag data from const functions.
"ItemListMap" has been renamed to "ItemMap" (with the old version
deprecated). It seems that the "ListMap" notion was copied probably
from Allan's ApeTag implementation, which incorrectly copied the term
from the XiphTag. Notably, in XiphTag, because a field can have multiple
values, the "ListMap" is a map of lists. Calling things a "ListMap" where
there can be only one value doesn't fit.
Closes#255
bba562b557