New long-range pathfinder.
Based on Philip's work located at http://git.wildfiregames.com/gitweb/?p=0ad.git;a=shortlog;h=refs/heads/projects/philip/pathfinder
Includes code by wraitii, sanderd17 and kanetaka.
An updated version of docs/pathfinder.pdf describing the changes in detail will be committed ASAP.
Running update-workspaces is needed after this change.
Fixes #1756.
Fixes #930, #1259, #2908, #2960, #3097
Refs #1200, #1914, #1942, #2568, #2132, #2563