The proper way to add custom tiles?

Hi!
I want to build my own tileset, and I know that it is possible, though I have no idea how to do that. To use my own tileset I have to build .strf file and inside tiles.strf file it is mentioned that: "the meaning of the values in (attributes ...) can be seen in src/tile.cpp, unisolid is 3 not 2", but there is no such file in that folder.
My version of the Supertux2 is current stable 0.5.1
I want to build my own tileset, and I know that it is possible, though I have no idea how to do that. To use my own tileset I have to build .strf file and inside tiles.strf file it is mentioned that: "the meaning of the values in (attributes ...) can be seen in src/tile.cpp, unisolid is 3 not 2", but there is no such file in that folder.
My version of the Supertux2 is current stable 0.5.1