Class Browser list expanding during search; add delayed filtering

Summary:
This patch provides expanding of Class Browser list during search. Old version collapses classes list by default and don't expand it during search, user must do it manually, which is annoying action.

The patch also adds delayed filtering (500 ms. after the end of typing) which improves performance for big projects (we shouldn't update the filter immediately after the text is changed).

Old behavior:

1. Open Class Browser - list is collapsed by default
2. Type something in search line
3. Classes list is still collapsed

New behavior:

1. Open Class Browser - list is collapsed by default
2. Type something in search line
3. Classes list is expanded (after small delay) which allows to see results without additional actions
4. Clear search line - classes list is collapsed again.

Reviewers: kfunk, KDevelop

Reviewed By: kfunk, KDevelop

Subscribers: kfunk, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D2929