Implement feature that allows tasks to get the global properties

[jeffkl]

This is an implementation of #4925 which allows tasks to get the current project's global properties.

I added an IBuildEngine6 and plumbed the global properties to the appropriate task hosts.

[rainersigwald]

Should accessing this emit a log event? I kinda want it to to minimize the "invisible input" problem.

[jeffkl]

It seems a little weird to log a message but it does sort of make sense. I had coded up something but I couldn't decide on the message as I'm not sure what it should say?

TaskA requested the current global properties.

Fetching the current global properties for TaskA

Fetched global properties for TaskA

TaskA requested the following global properties:
  Property1: Value1

If there's a message you like, I'll gladly put it in there.

[jeffkl]

/cc @AndyGerlicher

[AndyGerlicher]

I would say no on logging? If we want that maybe in the style of #4461 which was reverted. Logging the reads seems like a natural extension of that.

[jeffkl]

@AndyGerlicher I agree, let's add logging events later as part of the bigger effort to capture things that effect the build, what do you think @rainersigwald?

[jeffkl]

@rainersigwald gentle ping...

[jeffkl]

Thanks all!