Up-to-date alternative design to #450. Pure virtual modifiers are already supported, albeit requiring an explicit mapping. This PR only introduces assigning automatically a canonical mapping to vmod that remain unmapped after the usual vmod resolution.

I do not think we want this by default, at least not in the v1 keymap format, as it introduces breakage. However it could be activated by default in the v2 keymap format.

TODO:

• Ensure automated canonical mapping is made explicit at serialization
• fix Specify virtual modifier indexes assignment #755
• guard with v2 keymap format check
• update the tests
• update description
• Update the commit messages to reflect current implementation

(old description, require update)

Traditionally, virtual modifiers were merely name aliases for real modifiers, e.g. NumLock was usually mapped to Mod2 (see modifier_map statement). Virtual modifiers that were never mapped to a real one had no effect on the keymap state.

This patch introduces the concept of pure virtual modifiers, i.e. virtual modifiers that are not mapped now show up as if the were true modifiers.

Note that pure virtual modifiers cannot be used in an interpret action's AnyOf() and an interpret action for a pure virtual modifier must be AnyOfOrNone() to take effect:

virtual_modifiers APureMod,...;

interpret a+AnyOfOrNone(all) {
  virtualModifier= APureMod;
  action= SetMods(modifiers=APureMod);
};

The above adds a pure virtual modifier for keysym a.

Interestingly, this fixes one current issue with our tests: previously the de(neo) layout level5 didn't take effect correctly, with this patch in place it now behaves.