Overhaul of unit_movement_type

This is a general overhaul of the class embodying movement types, featuring:
* Better data encapsulation
* Less duplication of code between unit.cpp and unit_type.cpp
* Easier to use
* New files for the class (VC and XCode projects still need updating)
* New (shorter) name for the class
Some additional revisions will be coming.

The primary motivation for this was to get a class that embodies movement costs
(part of the data encapsulation).


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56348 75b3775d-3801-0410-9e12-e2e43121521d