QPageSize's static data tables are too large

We've already fixed the really low-hanging fruit in

• https://codereview.qt-project.org/c/qt/qtbase/+/394586 and
• https://codereview.qt-project.org/c/qt/qtbase/+/394651

which, between them, already reclaimed almost a whole memory page.

During review, further reduction potential was identified:

1. don't store sizes in pt, in, and mm, but only in one of them (todo: which?) and calculate the others on-demand (fails tests which expect exact numbers)
2. store FP data in float or fixed-point, not qreal (= double) (same problem)
3. adapt qOffsetStringTable so it can generate the string table separately, then we could move the const char[] member out of the struct and replace it with the quint16 offset copied from the offset table
4. verify that the ID is numerically the same as the table index and don't store it in the struct

This task was created so we don't forget about these ideas.