Minor adjustments

-Added more comments
-Edited some strings
-Minor code adjustments
Controlled SDL installation (+ other changes)

-Function check_for_sdl will prompt user to reinstall SDL in case the
“picked” SDL is already installed
-Moved partial SDL installation check into check_for_sdl (was in
function check_for_libfiles)
-Made intro string into variable ($introMsg)
-$SDLPicked ===> $pickedSDL
-Added & edited comments
-Minor code adjustments