WebAssembly: Initial commit

This change adds WebAssembly support in the shape of a plugin.

- Auto-detection of the emsdk toolchain
- Handling of "asmjs-unknown-emscripten" Abi
- Binary detection of WebAssembly libraries
- Auto-creation of a "WebAssembly runtime" device (with icon)
- Runconfiguration that launches the application via the "emrun"
  tool which spawns a local web server and runs the application on
  the chosen web browser.

Limitations:
- So far only tested on Windows/MinGW and Linux
- Not yet tested with Qt WebAssembly installation form the installer
  Only tested with self-built Qt and manually added kit
- The attempt to launch an application via emrun, while a previous
  application is still running, will fail. The reason is that the
  web servers spawned by emrun listen to the same default port but
  serve only the content of one application.
  Possible solutions: We could either spawn the different web servers
  with different ports, or we could use one single web server instance
  which serves the whole default project location (home directory).

Task-number: QTCREATORBUG-21068
Task-number: QTCREATORBUG-22249
Change-Id: I1a16fbe52382d45c37e9bc624a943a6ca475fa09
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/plugins/plugins.qbs

@@ -74,6 +74,7 @@ Project {
         "updateinfo/updateinfo.qbs",
         "valgrind/valgrind.qbs",
         "vcsbase/vcsbase.qbs",
+        "webassembly/webassembly.qbs",
         "welcome/welcome.qbs",
         "winrt/winrt.qbs"
     ].concat(project.additionalPlugins)