Handle numeric fields properly in filter creation dialogs

Every numeric field, like length, disc number and track number, were
not properly parsed by the filter dialog or by the dynamic playlist
dialog into the textual query string used to then apply the filter.

The bug was due to what sounds like an "optimization" that swapped the
first and second value of a "between" condition, to make sure the
smaller value was always the first value and the larger always the
second. The problem was that this was applied also to non-between
conditions, actually swapping the lenth or disc number with 0, making
0 appear in the textual representation of the condition.

The fix makes sure that "optimization" is applied only in case of
actual "between" conditions, leaving the values "unoptimized" for any
other kind of condition (=, <, > and similar).

REVIEW: 128245
BUG: 341661