Added rudimentary support for xlink

Created by: hbekel

Here's roughly what I did: Added a Singleton called Preview, connect the signals fileLoaded and tileUpdated to it, Changed tileUpdated to pass the tileIndex. Then on fileLoaded, the Preview class loads the charset to $3000 and pokes $d018,#$1c to show it. Everytime tileUpdated is called, the 8 bytes of the updated character at tileIndex are loaded to the proper address, assuming for now that all tiles are 1x1 Character. If the remote machine can't be pinged, this fails silently.

The build file now expects ../xlink/xlink.h and ../xlink/xlink.dll to be present on windows (For Linux this isn't needed if xlink is installed, don't know how macosx handles it).

To use with vice, please see the xlink docs (export XLINK_DEVICE=shm and run x64 -xlink).