Better selection

Changelog:
I. body.lua:
	- new centerX,Y, Vx,y "getters" and "setters"
	- new useful ispointinside function (may change name)
	- changed move_to to use :normalized() (may go back to atan2, sin and cos
again)
II. character.lua:
	- radius is no more (you can actually use it but it will be changed to a
normal size)
	- position is now top-left (instead of center)
	- each character has it's own 'marker' (better for when there are two
distinct destinations, also, relieves selector of some work)
III. selector.lua:
	- no more 'global' marker
	- better selection: you can now click to select
	- prettier selection box

TODO: Add ctrl-select