Cppcheck: add entry to project tree contextmenu only if supported mimetype

Test Plan:
Invoking the context menu (e.g. by RMB) on a C/C++ file still has an Cppcheck
entry in the "Analzye with..." submenu. Other files no longer do.

Reviewers: KDevelop, antonanikin

Subscribers: kdevelop-devel

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D14525