Call new shell api setModal to enforce the modality of a JS dialog.

[RaymondLim]

This pull request goes with another one in brackets shell adobe/brackets-shell#268.

Also remove all the hacks that we put in when switching to native menus. So after this pull request landed, we need to regress #2616, #3152, #3219 and #418.

[njx]

@RaymondLim @gruehle - looks like we never merged this and the associated shell PR. I have a vague recollection that we had some concerns with it, but it doesn't look like anyone ever commented on why. Do you guys remember where we ended up?

[gruehle]

We wanted to investigate general command enabling/disabling to make sure it didn't conflict with this pull request.

We discussed this at an architecture meeting. I don't remember coming to any conclusions about command enabling/disabling, but I'm pretty sure we decided that a modal switch would not conflict. @RaymondLim does that sound right?

[dangoor]

@RaymondLim I think we should probably close this PR. If you'd like to get this in, I'd recommend rebasing with current master and submitting a separate PR.

Does that make sense to you?

[RaymondLim]

Closing for now.