- changed the directory search for instruments back to only searching inside `getInstrumentDirectories()`
- created a new `FileDescriptor` function `isXML()` to determine if a file is of XML type (currently this is true if the file is Ascii and has an ".xml" extension)
- made `FileDescriptor.extension()` to always be lowercase
- created new functions `toLower()` and `toUpper()` in `Kernel::Strings` to convert input strings to lowercase/uppercase
- added a test for `LoadEmptyInstrument` where `LOKI` is loaded from instrument name