* Modernize lua read (part 2 & 3): C++ templating assurance
Implement the boolean reader
Implement the string reader
Also remove unused & unimplemented script_error_handler
Add a reader with default value
Adds the possibility to colorize item stacks based on their metadata.
In the item/node definition you can specify palette (an image file)
and color (fallback color if the item has no palette or metadata).
Then you can add palette_index to the metadata.
Dropped itemstacks with different colors do not merge.
* mod can create a ModMetadata object where store its values and retrieve it.
* Modmetadata object can only be fetched at mod loading
* Save when modified using same time as map interval or at server stop
* add helper function to get mod storage path
* ModMetadata has exactly same calls than all every other Metadata
