Debugger: Allow loading dumpers from a file directly

Instead of requiring the user to add the required python
code to load them manually let him specify a file directly.
This also makes the feature accessible to LLDB which needs
a slightly different syntax.

Task-number: QTCREATORBUG-12492
Change-Id: I12621b1cea2429476b0865bcbcb739a238dcf6af
Reviewed-by: Orgad Shaneh <orgads@gmail.com>