Enable "Freeze Selection" for non-Find-Data selns.

Change-Id: I861d3c65f2f10246594793d4a227011f606655f5
Fixes for modeless Find Data.

Rewrite `pqDataQueryReaction::showQueryDialog()` so that only a
single Find Data dialog gets instantiated, and do not call
`QEventLoop::exec()`.

Also, the dialog misbehaves when no source is selected in the
pipeline inspector: the list of field associations is not
correct and the arrays in the default query clause are always
point data (even though Cell is usually the default association).
This fixes the `pqQueryDialog` constructor by choosing the default
filter as the producer when NULL is passed.

Change-Id: I1566b58a568304077aaf2503861dd842414b3fa2
Merge topic '10968_freezier_frozen_selection-rebased' into next

74047c9 Fixes for modeless Find Data.
567b340 Enable "Freeze Selection" for non-Find-Data selns.