Allow moving map with left mouse button + alt key

[Zverikk]

Currently Alt key has no use on the map view. I propose making Alt+LMB an alternative combination for moving the map.

Of course, it should not affect Alt+LMB click for selecting overlapping objects and Alt+Shift+LMB for scaling objects.

This would greatly benefit pen tablet, touch pad and mac os users, who favor left mouse button over other buttons.

[Zverikk]

Also see #9897 — it'd be great to add similar functionality to the download window. Alt or Ctrl — doesn't matter, though there might appear some use for Ctrl key in the future.