-   avoid dialog box for EVENT.TYP<255;
-   remove branch for libbiosig older then v1.9.3

   When the base frequency of the event file and the data file do not match
   the event positions were wrong. Actually, FileSignalReaderFactory
   was overwriting samplerate (and perhaps also some other paramters).
   The event file is now opened directly (w/o using FileSignalReaderFactory)
   and converted into the event table.

Fixed SEGV when saving to CSV after deleting an event

Re-add support for importing/exporting events from/to *.evt

   this ensures that event files stored in 0.5.2 and earlier are readable again.
   (this was disabled since commit 88be2e34)

Fix link to external dependencies for win32

Clean Up main.cpp and main_window.h and cpp