Fix build select with multiple folders on Windows
Fix build select with multiple folders on Windows

In Windows, when you had multiple folders open, this would fail to match
the folder the file currently open resides in, causing it to be unable
to detect a pre-existing project.xml, for instance.

Using the platform-agnostic os.sep fixed this issue.

strauzen/haxe-sublime-bundle,MattTuttle/haxe-sublime2-bundle,MattTuttle/haxe-sublime2-bundle,clemos/haxe-sublime-bundle,joa/haxe-sublime2-bundle,strauzen/haxe-sublime-bundle