When starting program with at least one card file and a large number
of QSOs to be loaded during startup, the main window will flicker
rapidly and the Startup Screen shows through the main window.
This commit fixes this by:
1. Loading the card files before any ADIF files.
2. Closing the Splash Screen before showing the main window.
3. Adding a caution to the user manual to not load ADIF files
containing a large number of QSOs.
This fixes issue #39.