various fixes for "advanced" operations on range selections. ctrl-drags now add a...

various fixes for "advanced" operations on range selections. ctrl-drags now add a new range selection, allowing discontiguous selections as in ardour2, shift-click extends an existing range selection to the clicked position, alt-drag on a range selection moves it, and to do a so-called "separation drag" now use ctrl-alt-drag (or ctrl-alt-click)

git-svn-id: svn://localhost/ardour2/branches/3.0@13660 d708f5d6-7413-0410-9779-e7cbd77b26cf