We used a typedef "filter_preset_table_t" for the filter preset table,
because it is a "std::vector<filter_preset>". However, that is in
contrast to all the other global tables (dives, trips, sites) that we
have.
Therefore, turn this into a standard struct, which simply inherits
from "std::vector<filter_preset>". Note that while inheriting from
std::vector<> is generally not recommended, it is not a problem
here, because we don't modify it in any shape or form.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>