[WIP] Support for XDF

[cbrnr]

Prepare support for loading XDF files. Currently, XDF can't be loaded yet, but a dialog window shows a list of all available streams contained in the file. Marker streams (streams with fs=0Hz) are disabled. The plan is to load the selected stream (for now, only one stream can be selected and loaded). Marker streams will be imported as annotations.

[cbrnr]

A single XDF stream can now be loaded.

Caveats:

1. Large files slow down the workflow quite a bit. This is especially visible when plotting a large stream.
2. Works best with EEG streams. Other types can be loaded, but plotting an audio stream crashes Python, because MNE is not prepared to plot streams with such a high sampling rate (and therefore some heuristically chosen values cause a buffer overflow). This is e.g. the case for the sample_xdf.xdf file in the XDF repository.
   3. The model and view are not cleanly separated yet. This is problematic when opening an XDF file from the File - Recent menu, which currently crashes MNELAB.
3. Marker streams are not yet imported (as annotations).
4. Only one stream can be loaded, it would be nice to have the option to load multiple streams (in which case we will probably need to resample).

Despite these caveats (I will at least work on importing annotations soon), comments are welcome!