Fix bug in filter function when havin multiple filters

Since commit 707323067d62d7177a96418c8e1201297fc70d13 there has been a
bug in the SqliteTableModel::setQuery method which rendered the filters
useless when using them on multiple columns: Because this method is
called whenever a filter value is changed and because it deleted the
internal representation of the filters all filters except the one last
modified were deleted but still shown in the UI.