Allow add/remove reference within one of many target frameworks


In this multi targeting project, removing a reference from a single target framework currently removes it from all target frameworks.

This is probably what you want most of the time, but the user lacks the ability to remove from a single target framework via the UI.

Perhaps add a new menu item "Remove from .NETFramework 4.5".

The case of adding is more challenging, as the context must be carried through the subsystems that select references (e.g. Reference Manager and Package Manager).

@davkean pointed to [`IConditionalPackageReferencesService`](http://index/?leftProject=Microsoft.VisualStudio.ProjectSystem&leftSymbol=xwopgxg9nrqa&file=References%5CIConditionalPackageReferencesService.cs&line=37) in CPS which supports adding and removing with conditions.