LiveDevMultiBrowser should properly search parent directories for an index.html

Presently, LiveDevMultiBrowser gets stuck in an infinite loop trying to step into parent directories to find an initial index.html if none is present in the current directory. Without the fix, the included test will loop forever.
@busykai I think `getBaseName()` just returns the dir name alone, without any preceding path segments.  What we want here is the full path of the parent folder.  The LivePreview.js code this was copy-pasted from uses a [`getParentFolder()` helper](https://github.com/adobe/brackets/blob/master/src/LiveDevelopment/LiveDevelopment.js#L683-L685) to do this, with a TODO suggesting we should move it into FileUtils.  Maybe now is a good time to do that, so it can be shared with the code here.

While we're at it, we may also want to remove LiveDevelopment.js's copy of `getFilenameWithoutExtension()` which is 100% identical to the one in FileUtils (i.e. the version being called in this copy of the code).