Army size is now limited in class Army.
Added Engine::getMaxPlayerUnits().

Calculates the maximum number of units a player can posess depending on the size of the board.
Engine sets max army size when initializing.
Method name refactored: Army::getTotalUnitCount() -> getUnitCount().
Implemented Army::getTotalUnitCount().
Engine sets max unit count for army to INT_MAX.
Added sfg::Separator between unit and board set up in StartGameWindow.
Merge branch 'unit-limit' into dev

This branch was supposed to resolve #31. See this issue why it was not possible to do so ;)