Add ability to show ProblemsView with selected model data.

Summary: This patch provide mechanism for activating (showing) selected tabs in Problems View. This is useful for plugins, which provides custom problem models and allows them to automatically (for user) shows results of the work.

Test Plan:
Tested on new version of kdev-cppcheck pugin (not pushed to upstream yet):

core()->languageController()->problemModelSet()->showModel("Cppcheck");

Reviewers: mwolff, KDevelop, kfunk

Reviewed By: mwolff, KDevelop, kfunk

Subscribers: kfunk, mwolff, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D2634