Check name to determine whether node is map or cars

This approach is better than hardcoding the index.
Add getter method for the level number
Add overload constructor without redundant parameters

This change gives rise to the issue regarding the texture. Since the Texture
in LevelScreen class is only used as an argument, instantiation of the Texture
is done in the Car class. However, the Texture in the LevelScreen class is not
removed in case it is planned to use that variable in future commits. A TODO
comment addressing this suggestion is added.
Implement XML parser for cars
Temporarily change the parser to a public variable

This change is made because issue arises when instantiating two MapParser
objects (i.e. going over the initialization of the maps and traffic signals
twice) where cars stop at the first traffic signal they encounter. This issue
is probably caused by the traffic signals not being hooked up to the grids.
Thus, in order to make the reading of the XML file only once, the parser is
made available to the LevelScreen class by making the visibility public.
Change constructor for Car class

The number of arguments is lessened to increase simplicity.
Pass in the parser instance when reading map

This allows the use of only one parser object while not making the parser
public in the  VerilogTownMap class.
Merge branch 'xml_parse'

This is the topic branch for parsing the XML file.