Support drag and drop into scripts

[agc93]

We should allow dragging-and-dropping supported files directly into scripts.

My initial view of this should be:

• Support dropping .cake files and automatically #load them
• Support dropping .dll files and automatically #r them
• Leave existing behaviour unchanged (i.e. support dropping solution items as a file path)

This should leave the major use case of referencing per-project files supported, while adding the #load and #r behaviour.