Changing Platform target for one Configuration affects them all

Repro:

1. Start with the following csproj:
```xml

netcoreapp2.0
AnyCPU

```

2. In Project Properties, Build, with Configuration set to Active (Debug) and Platform set to Active (Any CPU), change Platform target to x86 and Save.

Expected:
Only the Debug configuration is x86; the Release configuration is still Any CPU.

Actual:
All configurations are now x86. The project file was edited to say:

```xml

netcoreapp2.0
x86

```

Note that I'm quite pleased that the UI doesn't unnecessarily add conditional property groups (such as when changing the Platform Target when Configuration = All Configurations). It just shouldn't use the unconditional property group when the active Configuration is a specific one and others exist.