Fix problems tooltip

Summary:
This patch fixes problems tooltips. Current version shows tooltips only for problems from built-in parser. New version provides:

1. Tooltips for all problems (IProblem interface) stored in any Problem Model.

2. "Multi-error" tooltips. One line can contains several problems, new version shows them all in one tooltip.

3. "Multi-error" tooltips are sorted by problem severity.

Test Plan:
Tested on master branch with kdev-cppcheck, kdev-valgrind, kdev-verapp plugins. All tested cases work correcttly:

1. "Single-error" tooltips (old behavior). Works with built-in parser and external plugins.
2. "Multi-error" tooltips without assistants.
3. "Multi-error" tooltips with assistants.

Reviewers: kfunk, KDevelop

Reviewed By: kfunk, KDevelop

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D3291