Add support for "persistent IDs" (the DCxx IDs)

[ahyangyi]

Some notes:

• The existing "global IDs" are limited to TRAIN; I did not touch them.
• The definition of persistent IDs have to come before any Action0 referencing them; since they are global, prepending the actions to the sprite list seems to work, but perhaps you want to insert them in other ways.

[ldpl]

What are you trying to do with those strings that can't be done with 0xD0xx? Also, iirc feature doesn't matter for 0xD0xx range.

[ahyangyi]

A few RoadType properties are strings. They can't seem to use 0xD0xx strings during my tests.

The documentation about 0xDCxx also states:

Unlike the D0xx GRF texts, these IDs can be used to set properties. The text ID must be set before any action 0 references it.

My current usage of such persistent strings can be seen here.

[ahyangyi]

Hmm, the dict-in-dict implementation might be wrong if 0xDCxx doesn't care about the feature tag either... I should confirm it.

[ahyangyi]

Confirmed and code updated.

[ldpl]

Confirmed and code updated.

Did you confirm that persistent strings ignore feature?

[ahyangyi]

I tested that RoadType Action0 can use persistent strings defined with grf.TRAIN

[ldpl]

Ok, I'm not really happy with global strings api but for now it will do. Eventually I'll try to make it so that you can just use string and framework will handle allocation on its own.