New construction logic, performance improvements, bugfixes:

- Prevent construction if item is blocked (overlapping another item). [Game.cpp]
- Prevent construction if item is wider than floor below/above. [Game.cpp]
- Extend lobby if it already exists on floor. [Game.cpp]
- Switch to finger tool after construction of elevator. [Game.cpp]
- Added item-maps (by floor, by type) to improve performance when accessing items within game. [Game.cpp]

Bugfixes
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- Adding of cars to elevator only when construction tool is within elevator shaft. [Game.cpp]
- Elevator cars are added at the floor where the construction tool is positioned. [Elevator.cpp]