Added floor item. Also, bugfixes to path finding

- Added floor item. Constructed and extended automatically with every construction of building item and resizing of elevator shafts. Rendered 1st before any other building sprites.
- Changed the construction code to use this new floor item to check floor width limits instead of iterating through all items on the floor.
- Added a workaround to render sky colour for the windows in the sky lobby, since the floor item is rendered behind the sky lobby. Rendering the obscured parts of the sky lobby floor transparent to show the actual sky background would be a better option, but this would only be possible in SFML 2.0 (using RenderTextures).
- Removed defaultCeiling() since floor item is now implemented. [Item.h, Item.cpp, FastFood.cpp]
- Use prototype icon instead of prototype id to check for item type (reduced inefficient string comparisons) [Game.cpp]
- Since only ground lobby can be built on floor 0, ignore finding routes on floor 0. Also do not display no-route sprite on floor items. [Item.cpp]

Bugfixes
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- Return empty route if start and end nodes are the same. [PathFinder.cpp]
- Changed the cost of travelling on an standard-elevator to include maximum walking distance cost, to prevent usage of elevator when stairs/escalators are within walking distance. [MapSearchNode.h]
- Reduced the cost of travelling on an express-elevator to encourage usage of express-elevators (with stairs/escalators) before standard-elevators.
- Properly estimate goal distance when starting node is a floor node but ending node is a transport node. [MapSearchNode.cpp]