Task completion inside targets

[KirillOsenkov]

Should maybe offer all available tasks inside targets:

at the very least, maybe some common ones, Message, Exec, Copy, Delete, RemoveDuplicates, etc.

[mhutch]

Oops, broke this by accidentally removing the isAbstract from the Task element :/

[mhutch]

I special cased the Message task's Importance attribute as well so it gets validation and completion, turns out it's strongly typed internally but stringly typed on the task property.