Shift+PgUp/PgDn was a poor choice for bookmark-browsing keys, as
it's also useful for selecting sync-data for copy/paste.
Fix this by changing the hotkeys to Alt+PgUp/PgDn instead.
BASS and SDL both add a set of headers and libs that needs
to be included into the project. Allow the user to put these
headers in a subfolder called "include" and the libs in a
subfolder called "lib".
