Add localization "foundation" (many code changes)

This commit adds most of the code changes necessary for making the game
translatable, but does not yet "unhardcode" nearly all of the strings
(except in a few cases where it was hard to separate added
loc::gettexts from foundational code changes, or all the localization-
related menus which were also added by this commit.)

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/src/KeyPoll.h

@@ -59,6 +59,8 @@ public:
     bool controllerButtonDown(void);
     bool controllerWantsLeft(bool includeVert);
     bool controllerWantsRight(bool includeVert);
+    bool controllerWantsUp(void);
+    bool controllerWantsDown(void);
 
     int leftbutton, rightbutton, middlebutton;
     int mx, my;