QML Project run configuration needs environment

Right now, there is no custom runtime environment that can be set for QML Project run configurations. This is needed because otherwise, you cannot set a custom QML_IMPORT_PATH and possibly other stuff that the user's custom C++ plugins might want to read.

An additional benefit of being able to create custom environments is that Qt Quick Designer can use the specified path to handle imports properly and more complicated setups like import paths in shadow build dirs would work too.