ISSUE-435: Forbidden font family names

In general no font family names are excluded from the mapping. The restriction of generic font family names is just made for the intermediate step to create a TVTT document that should be as close to a VTT document as possible. One goal is to be able to copy values of corresponding "features" where possible and perform the necessary transformation steps during "pre-processing".

The CSS property font-family and the TTML attribute fontFamily correspond to each other. Both support generic font-family names but some of them do not match. The TTML values that have no direct equivalent in CSS are:

* "default"
* "monospaceSanSerif"
* "monoSpaceSerif"
* "proportionalSanSerif"
* "proportionalSerif"

The value sanSerif has to be translated to "san-serif".

There is recommendation how to map some of the non-matching font family names in section "2.2.3 Pre-processing: Styling" -> Translation of Incompatible Values. Some mappings are not included because it was not clear how to map them at the time of editing.

There is one error in the listing of generic font-family names that should not be used. "serif" in TTML maps directly to "serif" in CSS. Therefore it should not be in the list of "restricted" TVTT values.

The values monospaceSerif and proportionalSansSerif could be mapped in the TVTT to the values suggested fonts in IMSC 1. They have no direct equivalent in CSS so it make sense to recommend a mapping.

Actions that could come out of this issues are the following:

* remove the values "serif" from the list of "forbidden" values
* complete the mapping recommendation ot not equivalent generic font-family names for the pre-processing step
* recommend mapping in the preprocessing step from proportionalSansSerif to arial and monospaceSerif to courier.