Add support for EEGLAB files

[cbrnr]

SigViewer should be able to open EEGLAB files, which come in two flavors: (1) either a single .set file or (2) a pair of .set/.fdt files. A .set file is just a renamed .mat file, which in the former case contains both the meta information and the signal data. In the latter case, the .set file contains only the meta information whereas the signal data is stored in a separate .fdt file.

So if the data is available within the .set file, it should be available in EEG.data. If this field does not exist, the data must be retrieved from a separate .fdt file.

I guess we could use the free matio library for this purpose.