Add BXML Explorer, to see a live graphical preview of BXML files

Add BXML Explorer under Tutorials, but show it under Demos (like Component Explorer).

Note that at the moment this tool is experimental, and for example the code view inside is read only (you have to change the file from outside, and do a Reload in BXML Explorer).

In 2.1, verify if move this (and Component Explorer, and maybe even others) in the Tools subproject (to be re-created), and verify if generate a dedicated war for it (other than the related jar.