Only generate autoscroll events while selecting

As gumix explains in the bug below, "Konsole keeps sending mouse-drag
reports in ESC[?1003h mode after releasing button outside window [...]
The only way to stop it is to get mouse back over Konsole window."

These mouse-drag reports are actually generated by AutoScrollHandler, to
support the use case of extending the selection by dragging the mouse
outside the display area.

So, only generate those reports while selecting.

Can be tested by playing around in vttest:

 - 11. Test non-VT100 (e.g., VT220, XTERM) terminals
 - 8. Test XTERM special features
 - 5. Mouse features
 - 7. Mouse Button-Event Tracking (XFree86 xterm)

BUG: 391975