Add global and local search for maps

[Seninha]

The game searches for maps only in the local directory: ~/.local/share/nsnake/levels.

It's a good idea to add searching for maps in a global directory, like /usr/share/nsnake.

Name precedence

For maps files that have the same name and are stored in different places (one locally and other globally) the local file takes precedence, so it will be loaded instead the one in /usr/share/nsnake

[alexdantas]

This seems good.
It would be nice to know what the other contributors think about it.

[ghost]

+1