Implement QQmlParserStatus in FolderModel

Summary:
This saves four calls to `invalidateFilter()` during startup with
default config. With non-default config it will save various
calls to `invalidate()` and `invalidateFilter()` more.

Test Plan: Tests pass with adjustments.

Reviewers: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9122