SigViewer 0.6.0

This new release of SigViewer adds support for XDF files. In addition, we squashed several bugs, tweaked the user interface, and added other enhancements. Important changes include proper support for missing data (missing samples are not rendered), export and import events from CSV files, auto-centering of channels around their means, a new icon theme, and the switch from Qt 4 to the latest version Qt 5.8 under the hood.