Change default selection keys

[hendrikbl]

An option do change the default keys for multiselection (shift) and box selection (ctrl) would be awesome.
Specifically i want to use shift + drag for box selection and ctrl + click for adding/removing nodes to/from selection. THe current behaviour is the other way round.

I achieved this behaviour (almost) by using custom keyboard and mouse event handlers, but it would be way easier to make this configurable via config.

The same goes for the positionFixedByClickWithAltKey option.