Add command line options and reselect wifi entry after rescanning

There was a todo in controlbox.cpp to make the number of seconds between WiFi scans configurable using a command line option, and also two other entries (counter_accuracy and counter_period). 

I have added command line options for all three in this PR. The only thing remaining is to add something in the man page about these options (and translation work if necessary). You could also change the shortcut options. 

Additionally, I fixed a related bug in ```52ac5e3``` that I had noticed before but became more obvious when I sped up the rate of wifi rescans. Whenever the rescan occurs, the tablewidget gets cleared and rebuilt so you lose your selection in the table. I fixed this by storing the current selection and re-selecting it if still available.