Line tool about grafx2 HOT 5 CLOSED

AFAIK, ergonomically it doesn't conflict with any other command.

Artifacts now fixed. Still need to make the straight lines thing...

Since r264, Shift+Click is now reserved for something else.

I removed the shift+click right mouse button emulation function from r272 as it 
is
already handled with apple+click (note that alt+click also gives the same 
result).

In dpaint, i remember it worked with the shift key which would lock the mouse
coordinates horizontally or vertically depending on the first movement made 
with the
mouse right after the shift key was pressed.

Fixed in r295