Add ProblemModel::setPlaceHolderText() method

Summary:
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