Add ProblemModel::setPlaceHolderText() method
New method clears current problems and add new "placeholder" item (problem) with selected properties. The method should be used to notify user about some events. For example, analyzer plugin can set placeholders at analysis state changes - started/finished without errors/etc.
The patch is used by D4816 revision, which fixes bug 375557 "No indication of cppcheck being run in background".
Test Plan: Tested with master branch
Reviewers: KDevelop, mwolff
Reviewed By: KDevelop, mwolff
Subscribers: kfunk, mwolff, kdevelop-devel
Differential Revision: https://phabricator.kde.org/D4884