With Qt >= 5.10.0 use QSortFilterProxyModel instead of KRecursiveFilterProxyModel

Summary:
QSortFilterProxyModel in Qt >= 5.10.0 can recurse when showing
filetering results, thus use it directly.

Test Plan:
Filtering with a string in the Projects panel shows all the matching
results including ones from any directories in the tree view.

Reviewers: Kate, cullmann, dhaumann

Reviewed By: Kate, dhaumann

Subscribers: kwrite-devel

Tags: Kate

Differential Revision: https://phabricator.kde.org/D20394