Fix order of selected columns in PVTransposeTable.

Selected columns were kept in a std:set<std::string> which
sort them in a lexicographic order that can break the order of
tables with lot of columns (col 1, col 10, col 100, ..., col 2, ...)
Now columns are kept in a std::vector instead.

Change-Id: Ic5df033a8f565db93ca9c9f71f38dab2d27a5436
Merge topic 'fix_transpose_table' into next

bc4144d8 Fix order of selected columns in PVTransposeTable.