rpm-ostree: Improve search and filter logic

More complete fix for https://bugs.kde.org/show_bug.cgi?id=468162

Whitespace at the beginning and the end of search keyword is usually not
relevant for the search field so trim it.

BUG: 468162

See also: https://bugs.kde.org/show_bug.cgi?id=470372

Cherry picked from: !575