Make tempo lines physical_screen_height high, add xml null check, Use clicked_regionv...

Make tempo lines physical_screen_height high, add xml null check, Use clicked_regionview directly to calculate region movement. Using drag_info.data may result in the wrong regionbiew being used, don't use canvas coords for region_drag_finished_callback x calculation (eliminates speed adjustment), prevent incorrect updates on dropped regions by updating canvas before placing region copies.

git-svn-id: svn://localhost/ardour2/branches/3.0@3965 d708f5d6-7413-0410-9779-e7cbd77b26cf