Key TextureHolder by string instead of enum

Loading textures will be accomplished by Lua/data files. These files
cannot reference an enum directly, so for ease of translation
TextureHolder uses strings to identify textures in memory. Overworld does
not load textures by default. It now exposes its loadTextures() method so
that clients can choose which files to load.
Load textures through Lua

LuaGameState exposes new interface function for loading textures into the
Overworld. There is none so far for the overall context. Startup.lua loads
textures then loads TMX file.