Add a new constant for qdoc mimetype to cpptoolsconstants.
Make the CppEditor handle qdoc mimetype.

This change has one user user visible effect:
Adding qdoc files to a qmake project will no longer put those files
into SOURCES where qmake will then try to compile them. They will show
up in "Other Files" instead.

Task-number: QTCREATORBUG-15872
Change-Id: Ibbf9af0a84fab59138f6b9ab41f1bba737b455c2
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>