[kdevplatform] FilteredProblemStore scope-fix

Summary: This patch fixes FilteredProblemStore - now it uses the value of current scope during problems filtration. Old version ignores scope value, which makes scope filtration GUI elements (provided by ProblemsView) completely useless.

Test Plan: Tested with standard ProblemReporterPlugin (uses own ProblemReporterModel) and with kdev-cppcheck plugin (uses standard ProblemModel). For both cases everybody works fine: grouping, scope filtration, 'show imports' filter.

Reviewers: mwolff, kfunk, KDevelop

Reviewed By: kfunk, KDevelop

Subscribers: mwolff

Differential Revision: https://phabricator.kde.org/D2571