Feat/missile class composition

[grantramsay]

• Missile attributes/characteristics are now specified in one place, and are initialised in the missile constructor (closer to proper composition).
• Missile attributes now use nested classes instead of friendship for private member access.
• Allow setting different speeds for missiles.
• Added a reference to missiles graphics on each level to avoid having to check every actor in world to find missile graphics on a level.
• Added town portal spell.
• Put missile max range within missile movement attribute

A lot of the changes are just whitespace from the "Tidy namespaces" commit

[grantramsay]

Will tidy this up and fix build later

[wheybags]

Thanks!