Contract Builder: Props - Dropships

[CWolfs]

Upgrade the Props system to include Dropships.

Dropships will have an extra property compared to Buildings/Structures/Destructibles, being:

• Starting State - this will be the starting state of the dropship (e.g. Offscreen / Landed etc)

The first stage will be to support only the vanilla dropships. Later expansions will support fully custom dropships.

A dropship is essentially a flying building in terms of game logic but some some extra logic related to dropship-specific logic.

Expansion on #578.

[CWolfs]

In develop.