Add Textbook

A relevant paragraph copied from the original commit history:

The idea is that we store all strings somewhere managed, and then the
hashmap only needs pointers to those strings. For storing strings, I
created a `Textbook` structure, which consists of one or more 50 KB
"pages" (allocated as needed) on which you can simply write strings in
both languages back-to-back with `textbook_store(textbook, text)` and
get pointers to each of them. (I was originally going to just use one
big buffer and realloc to double the size when filled up, but then the
hashmap would be full of dangling pointers...) When needed, like when
switching to a different language, an entire textbook can be freed at
once.

This commit is part of rewritten history of the localization branch.
The original (unsquashed) commit history can be found here:
https://github.com/Dav999-v/VVVVVV/tree/localization-orig

desktop_version/CMakeLists.txt

@@ -104,6 +104,7 @@ set(VVV_SRC
     src/DeferCallbacks.c
     src/GlitchrunnerMode.c
     src/Network.c
+    src/Textbook.c
     src/ThirdPartyDeps.c
     src/VFormat.c
     src/Vlogging.c