Retroterm v0.14

Mainly bugfixing release:

-Better, more robust exit to BASIC
-Fixed small visual glitch when using the split screen mode
-Disabled interrupts on command $A2, prevents crashes if
(re)connecting to a BBS while the split screen mode is
active.
-Turbo56K v0.6 implemented.
-Fixed severe bug with text output on early Commodore 64
Kernal revisions.
-Full digi-boost for 8580 SID.
-Intro screen exits automaticaly after ~2 seconds.
-Other small bugfixes.