CppEditor: "Follow Symbol Under Cursor" for virtual functions

F2 on a virtual function call presents a list of overrides in derived
classes. The function declaration of the static type is shown
immediately at the top.

Task-number: QTCREATORBUG-9611

Change-Id: I80ce906fa06272dc9fbd1662cd17500b8c77067f
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Do not generally add a plugin's directory to its include paths.

It's not necessary, and it lets people get away with using
the wrong kind of include statement.

Change-Id: I736a0aae93c930da37dbac548a3bb025838e7588
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>