Add event labels, fix bugs

1. Labels

• events snapped to a wrong position when added via mouse/keyboard, because sampling rate was not considered. It needs to be this way, since otherwise the positions are not correct when reloading events from a .mad_gui file.
• event labels can now be passed to the gui. Press and hold ctrl to add events
• region labels can be defined to include one or more event labels

2. Bugs

• fixed an issue with axis going off when having several monitors
• enabled using openGL and autodownsampling (see config.settings), which gives a high performance boost in loading and moving large datasets