WebAssembly: Add an options page for manual Emscripten configuration

At the moment, Qt Creator's WebAssembly plugin tries to autodetect the location of the Emscripten SDK. It puts up with the restriction that it does only work if the Emscripten SDK was activated without the --embed option.
But even if the SDK is activated without that parameter, there are reported cases where the auto-detection fails.

Let's add a Devices/WebAssembly options page, which allows for manual setup. The current autodetection can still be used to prepopulate the settings. Like with Android/McuSupport, the WebAssembly plugin could also show an info bar leading to the options page if there is a Qt for WebAssembly kit which lacks an Emscripten toolchain.