Warn user on shortcut override and speed up filtering slightly

[herronelou]

This addresses issue #2 and #11

For #11, I started doing a custom model + proxy model and view, but it got too complex with trying to add the delegates to have the widget in the table.
The gain was not worth the trouble, so I did the quick hack as described here: https://stackoverflow.com/questions/6785481/how-to-set-filter-option-in-qtablewidget

[dbr]

Thanks for this, looks good! Merged

I may add in an additional option to the warning dialog - being able to keep both shortcuts may be desirable in certain circumstances (e.g the same key can be used in both DAG and Viewer)

[dbr]

This looks good, thanks!

I've added an additional option to the confirmation dialog as there may be a case where you'd want to bind an already-in-use key (but most of the time replacing the existing one is the right thing to do)