Adds a subscription to screen-lines-changed

When the minimap is created, the minimap view changes the editor width,
and build it self on an outdated version, the `screen-lines-changed`
event allow us to be notified when the editor have updated the screen
lines after resize.
Moves opacity to lines

It will allow us to control more precisely the styles of other minimap
elements (I think to din & replace matches, and all the other stuffs
the minimap will handle)