API changes to allow for more mounting styles

API changes to allow for more mounting styles

NODE NAMES:

The base name of a node can be whatever you want.  The basic wall sign
will keep the node name you give, while its hanging, on-pole, and yard
variants will, where applicable, use the same name, with "_onpole",
"_hanging", or "_yard" appended.

One exception:

If your node has "_wall" in its name, that bit will be stripped-out when
generating the above node variations.  For example:

"default:sign_wall_wood"

...will become...

"default:sign_wood_onpole"
"default:sign_wood_hanging"
"default:sign_wood_yard"

MODELS:

The base sign model and entity files must now be named:

sign:   mymod_foosign_wall.obj
        mymod_foosign_facedir_wall.obj

entity: mymod_foosign_entity_wall.obj

their other variants, where applicable, must be:

mymod_foosign_onpole.obj
mymod_foosign_hanging.obj
mymod_foosign_yard.obj

mymod_foosign_entity_onpole.obj
mymod_foosign_entity_hanging.obj
mymod_foosign_entity_yard.obj

For `facedir` signs, the "_facedir" in the base sign model filename is
automatically stripped from the name when generating the entity model
filename, as entities don't have a concept of wallmounted/facedir/etc.
For example:

"mymod_foosign_facedir_wall.obj"

...will become...

"mymod_foosign_entity_wall.obj"

* allow passing custom onpole, hanging, and yard sign models, for
situations where the filename(s) thereof can't be derived from the base
sign model filename.

* allow passing a custom pole mount, hanging, and yard sign stick
textures as the third, fourth, and fifth tiles{} entries, respectively
(independent of the above model file option)

* fine-tuned the pole mount on the standard sign models to eliminate
overlapping faces (so it looks better with basic_signs' glass sign.

* Move basic_signs wooden yard sign back to signs_lib.