New build config by default uses existing build directory of other config

1. Create a new project.
   I used a Qt Widgets Application. It doesn't matter whether it's using CMake or qmake.
2. Go to "Projects" mode, "Build" settings.
   A build configuration was created:

   Name	Build Directory
   Debug	<src>/build/Desktop-Debug

3. Add a new build configuration with the type and name "Debug".
   Creator detects that this name is already in use and correctly adjusts the values:

   Name	Build Directory
   Debug2	<src>/build/Desktop-Debug2

   This was fixed with QTCREATORBUG-31470.

4. Rename the build configuration to "Debug1"
   Creator changes the name, but not the build directory:

   Name	Build Directory
   Debug1	<src>/build/Desktop-Debug2

5. Add a new build configuration with the type and name "Debug" again.
   Creator detects that this name is already in use and adjusts name and directory, but it is missing the already existing build directory:

   Name	Build Directory
   Debug2	<src>/build/Desktop-Debug2

   So build configurations "Debug1" and "Debug2" will now both use the same directory which will cause trouble.

It might make sense to also move the build directory when renaming in step 4, but that might mean that everything has to be built again. Creator should watch out for already used directories in step 5.